Resident Evil 6 is a hit-and-miss kind of game. It's certainly no 4.5/10 as Gamespot gave it. I'm roughly half way through Leon, Chris and Jake's campaigns. Leon's is the best by far. Jake's isn't bad... there's a lot of silly things that bring the whole campaign down, namely the pseudo-stealth Ustanak part which seems to cheat you at every turn. Chris' campaign is horrible... it's like third person Call of Duty with spider zombies. It's pretty much a brain-dead Hollywood action movie, and it really doesn't work well with limited ammo and a crappy cover system. I haven't tried Ada's as apparently it has little to do with the overall story.
I'd give it an 7.5 if you can ignore the fact that it's called Resident Evil. If you're one of those people that believes that the series should never change or evolve and should be stuck doing the same thing it always does time and time again, then maybe a 6. After going back and playing Resident Evil 2 and 3, I can't understand the latter group of people. I can understand the want for a quieter, scarier, slower-paced game... one with no AI partner which seems to be a staple of the series now, but how bad do we really have it?