Before I start, I'd like to say I've never played a Resident Evil game, so this is my first experience with it. And all I can say is wow. Capcom really made a solid gold game here. Here's the skinny:
Co-Op - Incredible. Everything - from the start to the finish - is masterfully done. When I first played the demo, I thought it was kinda disappointing that you can't move while you shoot, but despite that fact, I still managed to have a blast. With a buddy along for the ride, it's nothing short of astounding. Working as a team is amazingly fun - planning out a route to get past a certain area, then executing that plan, succeed or fail, is so much fun. I do recommend a mic, however, because otherwise it's somewhat hard to convey messages and work together. I wanted so much more after it ended, although it still gave me quite a lot. This game is going to stay on my Top 5 for a while.
Single-player - Didn't really have a lot of experience with single-player. I did play a couple levels though, and here's what I gathered:
The AI for your partner could use some work. Some of the things it does can be annoying/frustrating, such as running into the midst of enemies to grab some ammo. It also won't help you if you're caught by an enemy, unless you press the button to call her, even if she's standing within 2 inches of you. With a buddy, this is made much easier, as you can simply tell him, or he can see for himself, and come help you. Other than that, it was pretty good. Although after playing co-op through the entire game, it felt dull. Just isn't the same. If you're used to single-player though, then it should be just fine. I know a few people who loved it just the same, despite playing single-player.
Storyline - Great story, although I think long-time fans of RE may find it better than I do. As stated earlier, this is my first Resident Evil, so the returning characters and references didn't affect me as much. They do do a good job of filling in to the players what happened back in the previous games(in some very cool cutscenes I might add). There's also a 'History of Resident Evil' in the Library that has a timeline explaining everything.
Sound - Great. The screeches of the enemies to the voices of the characters to the sounds of the environment all work together flawlessly.
HD - Superb. There are some parts where it's quite unrealistic, but for the most part it's fantastic. Some of the cutscenes are mind-blowing. I was saying 'woah' through a lot of them, especially the fight scenes. Definitely worthy of the PS3's capabilities.
Controls - Can take some getting used to, but not bad by any means. In fact it was kind of refreshing to have a new control set, as I was getting tired of the same thing over and over. Some of them I could do without, but there are preset controls that you can switch to if it's too hard.
Length - Personally I played on Veteran my first time through and took my time, and got quite a lot of play time. Going back on easy, however, me and my buddy got through levels in minutes, because we were just rushing. It all depends on what mode you play and what you plan to do. If you just want to get through it - no collectibles, no items, etc. - you could beat it in..oh, 5 hours? If you're like me and want to get the most out of it, I'd say you could get at least double that, if not more. I believe I got more than 10 hours out of it, but I can't say for sure just how much.
Replayability - Probably the most replayable game I've ever had the pleasure of playing. There are tons of things to unlock by playing through more than once, and taking said unlockables along to get OTHER unlockables makes it that much more fun. All your items and weapons are saved, regardless of what you do. If you get halfway through and want to go back to a previous level on a lower difficulty? Go ahead. All your stuff is still there. Even if you're the partner in co-op for someone else's game, your stuff remains, making it much easier to progress. With a ton of weapons to upgrade, infinite ammo to unlock, figurines to buy, emblems to destroy, and more, this game will stay in your PS3 for a long time to come.