It can be fun to play the "tank" sometimes, charging into battle with no thought for your own safety and clearing a path with a wave of your hand. A walking tank is essentially what a space marine is in Warhammer 40,000, or at least it is in the lore. The way a space marine plays in the game is defined by the Space Marines Codex, a subset of the game rules that assigns values to all the usual game attributes (such as Strength, Toughness, Wound count, Objective Control, and so on.)
The beginning of the book provides great information about the force of space marines is organised as a whole, with data about specialised units and roles (like Chaplains and Librarians), Terminator and Dreadnought armour, vehicles, and more. This takes up a full 42 pages. Combine that with the 2-page spread on heraldry and field markings that helps you paint your army, and you've got a surprisingly complete lesson on the faction from both a lore and pragmatic view.
But of course there are lots of space marine chapters. It would be impossible to encapsulate lore developed in countless Black Library novels in this humble codex. As a result, each chapter gets exactly 1 page in this book, which amounts to 1 paragraph of history, 1 paragraph of current events, and a few paragraphs about how this one is definitely the best chapter of them all. It feels strange that one successor chapter (Black Templars) is included in a section otherwise containing only founding chapters, but I guess Black Templars are popular or unique enough to warrant it.
All in all, the lore section starts strong, and then becomes just sufficient. I definitely feel like there's room for a second volume of this codex with a lot more information about each founding chapter, along with an example successor chapter.
There are several pages dedicated to successor chapters (and chapters with unknown lineage). Each successor gets a photo and a brief description. It's one of my favourite sections because it's inspiring, and reaffirms that it's fully expected that your army is unique from the "official" chapters. Personally, I'm not terribly interested in developing my own look for a space marine chapter. I just don't have the fashion sense for it. But I do like being reminded that small variations in how I paint my models can always be explained away by declaring my army as a successor chapter, or just by remembering that this is all pretend anyway.
The additional lore about each successor, brief though it may be, is also particularly inspiring. Like the paint scheme in the photographs, the simple description of a successor chapter is a gentle reminder that your custom army's lore isn't going to start as a Black Library novel. All you need is a cool chapter name and a one-line description, and that's enough to start from. As you play the game, your army's lore develops itself. The army lists you find yourself defaulting to, and the tactical patterns you fall into based on either what you find fun or what you're forced into by the tides of war, become part of your chapter's story. It's a major component of playing Warhammer 40,000, and it's embodied in just a few pages of this codex.
A space marine is mostly armour, so the colour you choose for your army is the most pronounced feature. They're clearly identified by whatever colour scheme you create.
Because the design of space marine miniatures features a lot of flat armour panels that are pretty easy to paint. You can do a base coat of a single colour, then a layer for belts and straps, and a final layer of grey or black for weaponry, and you're done. It's the easiest painting in the entire hobby, I think, and it must be one of the reasons that space marines are promoted as the premiere faction of the game.
The showcase section of the codex isn't just a series of photographs of beautifully painted miniatures and battlefield setups that are both impractical and basically impossible to achieve. Instead, you get examples of popular unit types painted to a battle-ready standard so you know what to aim for when you sit down to paint your army. It's easy to say that Blood Angels are red and Salamanders are green and Ultramarines are blue and yellow, but the devil is in the detail. What about the insignia on the chest plate? What about the pauldrons? What about the helmet? Look at the showcase and you can see for yourself.
For whatever reason, the showcase stops short of providing a paint list. There's exactly 1 paint list on page 96, and it's for the Ultramarines because they're the chapter featured in the Combat Patrol section. It seems like a missed opportunity to just list the Citadel paints you need for each chapter, but I guess by not listing them the book grants you implicit "permission" to use whatever shade you prefer. In a pinch, you can probably just go to your local Warhammer store and ask, or probably use the Citadel Paint app, but personally I'd have appreciated having the info in the book. Realistically, I'm just going to go write it in myself, so it would have been nice for it to be included already.
Combat Patrol is the format of Warhammer 40,000 allowing for small armies. It's more than a skirmish game, but smaller than a full battle. If you want to play a faction, you have only to buy its Combat Patrol box to try it out. Of course you still need the codex and the core rulebook and the paints, but Combat Patrol is a nice starter or taster army. There are other ways to play Warhammer, including actual skirmish games like Kill Team or Necromunda, but if you want the literal 40k experience without investing in a big army, then Combat Patrol is the solution.
I'm a fan of Combat Patrol, and in a way it's mostly what I actually play. Keeping armies relatively small is a nice way to make the game manageable for solo play, or conversely for multi-player games that don't take half a day to get through. Skirmish is the obvious answer to both problems, but there's just appealing about moving units around the table instead of individual soldiers.
The Combat Patrol detachment in the codex is Strike Force Octavius, and it contains a Terminator squad, a Librarian, a captain, with a unit of marines with flamers. The datasheets provided for each unit contains only the wargear included in the box, so the Terminator in Combat Patrol has just 4 weapon lines to parse instead of the 8 required to account for all options in a box of Terminator space marines.
If you're buying the Space Marines Codex for the rules, then this book is absolutely worth the cover price. This is the book with the special rules and numbers you need to play space marines on the tabletop. There are some specialised codexes for specific chapters, but this is the book you need for a general purpose army of Adeptus Astartes.
The detachment section provides several ways to field an army, and then to take advantage of your choices. All detachments are technically generic. You can paint your space marines any colour you like, call them any chapter or a successor of any chapter, and then use any detachment in the book. All options are valid, or at least that's the theory.
In practise, a few of the detachments feel more generic than others. The Ironstorm Spearhead, with its Adept of the Omnissiah enhancement, feels obviously like an Iron Hands detachment, even though technically you can pop a Techmarine into any chapter and use the detachment. The Firestorm Assault Force is obviously a Salamanders detachment, even though (as the Combat Patrol demonstrates) you can stick a unit of Infernus Squad into any chapter and use this detachment. Every detachment, to some degree, evokes a specific chapter while simultaneously being generic enough for use with any chapter.
That's not a problem, but it does somewhat remind us of the conceit of the space marine faction, and indeed the entire game. Our plastic toy soldiers aren't actually mechanically unique in any way. They only feel unique because we make up stories about them, and give meaning to the numbers we jot into our game notebook. For anyone who can't decide what chapter they want to adopt, it's actually liberating. The chapter doesn't matter. All that actually matters is that you enjoy the game. At worst, you might have to go buy a techmarine, or pretend that those bolters are flamers, but if you don't like one detachment then you're free to just try another.
I've played a lot of Warhammer 40,000, but the most memorable games have always been campaigns. I'm happy to play a one-off game of 40k, but given the chance I almost always connect one battle to the next. In official Warhammer 40,000 terminology that's a Crusade, and it's part of the Crusade subsystem that you upgrade your standing army after each battle in a campaign.
There are generic Crusade rules available from the Warhemmer Community website, but the Crusade section of each codex provides options specific to the faction. The space marines Crusade system includes Honour points, which your army gains by fighting in battles and by achieving bonus agendas. The section defines several agendas you can try to accomplish, but they're usually something far afield from the goal of any typical mission. You can either send a unit to go accomplish a "secret" mission (such as destroying an enemy soldier within a specific distance of an objective, or getting one of your soldiers within some distance of the enemy's battlefield edge, and so on), or you can play the game exclusively to achieve your bonus agendas. When you earn 5 Honour points, you can spend them on upgrades (or trade excess points in for experience points.)
The Crusade rules are relatively modular, so you can exclude whole systems without ever noticing. For my style of play, that's too much to think about during a game, so I usually end up ignoring agendas.
What I enjoy most are Battle Traits and Battle Scars. For each battle your army fights, you gain experienc points (XP.) You can spend XP on datasheet improvements (Battle Traits), such as an additional inch or 2 on your Move speed, or +1 Toughness, or +1 Strength to a weapon, and so on. Your army becomes more powerful the more you play, and it's hugely satisfying to watch your army become the legendary force you've always dreamt of.
Roleplaying games and wargames alike often "sell" you on an archetype. You read lore about a powerful hero who knows no fear and never loses a battle, so you decide to play that hero on the tabletop. And that's when the dream dies, because dice are fickle and games with no risk are boring. Your hero isn't as tough on the tabletop as the novels or flavour text claimed. It can be frustrating, and of all the Warhammer 40,000 factions available, I think the space marines suffer the most severe disparity. If you buy into space marines because they're portrayed as invulnerable super soldiers, then the mechanics of Space Marine Codex is liable to disappoint. With Tough 4 and a 3+ Save and 2 Wounds each, they're more resilient than Adeptus Mechanicus, but they're invulnerable.
Then again, neither are space marines in the lore, we only remember them that way. In the books and Warhammer+ shows, lots of space marines die. The thing is, we only bother remembering the ones that live. Those are the exceptional ones, and when you play Crusade you discover those exceptional soldiers within your army. You find the one that survives, and mark the underside of his base with a dab of paint, and grant him a name.
The point is, not every soldier can be a hero worthy of books and shows, or a special place in the plastic bin where you store your army. Don't let the stats get you down. Play lots of games, find your army's character model, and develop your own lore.