Yeah, I'm definitely enjoying the game, even with some odd chapters and design choices here and there. Co-op's definitely making it a better experience, too; it's nice to complain to your partner and laugh. I also agree that Mercenaries is worth the price of admission, but I would include everything that falls under Extra Content.
Mercenaries is in a league of its own as the best additional mode in any game, really, so when you add an extra No Mercy mode for it where it's double the total enemy count and you have insane numbers of enemies on screen at once, it gets so much better. Onslaught mode is no slouch, either; it's definitely a great and fun mode that gets you pumped. It being a weird mishmash of pure RE6 gameplay with battle puzzler mechanics wherein you send combos over to your opponent in the form of a variety of enemies makes for some intense matches (and makes me giddy), especially since health can be scarce and there's so much that can go wrong if you're not careful.
I'm genuinely and curiously excited to see how the other modes stack up once they're released. To Onslaught's benefit, it's a 1v1 mode, which makes it easier to get a game going; the others all seem to be 6 players, which could make it harder if the community's dead (which I hope doesn't happen too fast). I haven't even tried Agent Hunt.