Resident Evil 6 at its best is a varied and fun co-op experience that will keep you guessing what sort of scenario you'll find yourself in next, with some of the best controls in the series and some legitimate fun and tense segments. At its worst, it is a confused mess of ideas that tries to be everything, but doesn't quite refine or completely solidify any of its concepts. I almost reckon Resident Evil 6 to a grab-bag. It throws a lot at you; one moment you'll be in a ruined neighborhood in a zombie Apocalypse as chaos springs around you, next you'll be near-naked maneuvering a completely-white environment while stealthing to avoid setting off alarms, then flying a plane Ace Combat-style over the ocean, or scaling the side of a building as a colossal monster climbs right after you.
If you're the type of gamer who enjoys sometimes complete nonsense with a friend, but also the variety of situations appeals to you, this will be right up your alley. It is a fun co-op game, the combat isn't well explained and can be frustrating at first, but once you begin to 'get it', it actually is rather in-depth, and Mercenaries is a lot of fun (the most similar comparison I can give is to the game Vanquish, if you have played it, as the combat focuses a lot on dodging gracefully and sanctification from pulling off insane stunts by risk and reward). There's a lot of content here, with four campaigns (each completely different) that last 5-8 hours each, as well as an addicting Mercenaries Mode and a rather throw-away but still good for a few laughs mode where you control the monsters in other peoples games during certain gameplay segments.
But Resident Evil 6 is riddled with problems in its design, ranging from poor level-design, missing the sort of weapon upgrade system the last few 'action' RE titles incorporated to allow one to pick weapons by choice, the boss fights range from okay to dreadful (they're often more cinematic flairs than strategic or actually fun battles, which I felt RE4 and RE5 had), and simply some segments of the game just are not fun. And if you don't take the time to adjust to how the game plays (its pacing, controls, and the like on face value appear to be very much in-light with RE4 and RE5, but actually playing the game like those games isn't a lot of fun as the game is designed quite a bit differently) it is very possible you will not enjoy your time with it. It should also be mentioned that the story is terrible. Not cheesy B-fun like RE5 and RE4, not so-bad-its-good, but most of the story is just bad. I did enjoy some development in Chris's story, and the relationship between Jake and Sherry is cute, but outside of that, the basic 'plot' to the story almost feels like a fanfiction to the series.
But still, I had a lot of fun with it. Something about never knowing what to expect, going from one ridiculous scenario to the next with a friend, and coming to terms with the game and actually feeling pretty awesome fighting off a fairly large array of enemies in Mercenaries Mode was worth it for me, and for those who just want a good, often ridiculous time with a friend, Resident Evil 6 can deliver some good laughs and some high peaks in spades. Just expect to waddle a bit through some drudgery.
PS. I find a lot of people like different moments of the game the best, for the campaign, I will say my personal highs were (in no order) Leon Chapter 1, Leon Chapter 2, Jake Chapter 3, Jake Chapter 5, Chris Chapter 3, Chris Chapter 4, Ada Chapter 1, Ada Chapter 2, and Ada Chapter 4.