Resident Evil 6 has a lot of problems, like a lot of problems, a LOT of problems. BUT there's a lot of dumb fun you can have in RE6 if you let yourself. The game is a train wreck, it's such an uneven experience but it's kinda' fun to go through with a co-op partner through all of RE6's crazy and wacky shenanigans. Some of the level design is actually atrocious, but others times it can be surprisingly well done and with interestingly varied scenarios. RE6 is a content-packed full game with lots of variety. I don't think I've ever played another game that flip-flops so much between good and bad ideas and execution, and that's kinda' wild and fun to go through with a friend. There are actually moments of RE6's campaign I love, and other moments I actually hate. But I always have fun playing it with people, especially those who have no idea what they're in for.
Mercenaries in RE6 is a lot of fun, and I think I may be one of the only people on the planet who's played a decent amount of those DLC multiplayer modes with friends (full parties even, convinced friends to play it with me, have had a lot of dumb fun in those too). I will say this, I played the game first on 360 pre-update, and have ever since then post-update on PC, and the PC version post-update does add so much to the game experience, like it running 60fps with adjustable FoV and some of the fixes they apply did make me enjoy it a lot more.
Also, Leon's campaign is the worst one though I enjoy parts of it (and I love Leon's Chapter 4), Leon's actual story is also terrible, like it's baaad. Jake's campaign has some of the lowest moments in the game, but I kinda' love Jake's Chapter 3 and Chapter 5. I also expected to hate Jake before release, but I don't. Chris' campaign is the most solid all around and also has the best story of them all. Ada's campaign has it's moments, but also falls off a cliff with her Chapter 5 being like the worst of the worst in all of RE6. Also nearly every scene in RE6 involving helicopters SUCKS. Like the moments with helicopters in RE6 are among the worst in the game, and there's quite a few of them.
But that's where I sit on with RE6. It has something of a learning/enjoyment curve, I wasn't really feeling it when I first started playing. I did some Mercenaries to learn the mechanics back when I played it the first time, and that did help me get a feel for the game and start to enjoy the main game more. The main game still has a lot of flaws even with this, but I can say I've had fun playing through it a few times with friends.