Resident Evil 5 is the second Resident Evil game I've played. The first being Resident Evil 4. This is more due to my sheltered childhood then personal choice. I'm going to review RE5 as a standalone game and as sequel to RE4 since they are very similar games and share a common theme and universe.
Resident Evil 5 is less of a survival horror game and more of an action game where you continue the story of Chris Redfield fighting Albert Wesker. This time you have an AI partner named Sheva Alomar (although it is possible to play as Sheva and have Chris be your AI partner, however this changes little). RE5 tries to keep you in the action and does that with good pacing, fun weapons, and an real-time inventory system that allows you to interact with your partner to switch weapons and items. It's very basic though and only allows you and your partner to carry 9 items at once. You can find almost all the guns throughout the game on one play-through, but thanks to the upgrade system (which allows you to upgrade firepower, fire rate, reload rate, and ammo capacity) you can replay the single-player campaign many times, and make harder difficulties like Professional Mode easier. RE5 has mercenaries mode just like in RE4 but with more characters (i.e. different skins of 4 characters with different loadouts) and more maps which gives you points towards unlocking bonus content. The way to bonus features such as infinite ammo for each gun is much easier than RE4. You use points that you can earn in the single player (higher difficulties give you more), mercenaries, and Versus mode. Yes RE5 has a multiplayer component that costs about 800 points to download (I don't understand why they could not have included it for free). Versus is 4 player FFA or 2v2 Mercenaries mode that you can also win by shooting enemy players. This sounds really simple, and sometimes ends up being a kind of real-time strategy game, but if done right you can win games easily. Versus is also a fun way for players who played co-op campaign to get some more gameplay out of it.
All these things make for an excellent game that will give you much replay value. It makes many significant changes while still continuing to stay true (for the most part) to past Resident Evils. RE5 is definitely a real sequel that is different enough to be it's own game, but it strayed a little from its roots and doesn't really have as deep a single-player experience as RE4.