All I've played is the demo. Leon's campaign appears to be a step up from RE5 to me. While it lacks the white knuckle intensity and thrill-ride addiction of RE4, it does feel like a horror game and has a menacing atmosphere (though I'm not convinced on the new aiming style).
However, I can't stand Chris and Jake's campaigns. I like seeing Sherry Birkin back and the idea of her and the son of Albert Wesker teaming up to stop their fathers' horrid legacies is a cool idea, Jake's game feels like it is going to be a poor man's Uncharted and Chris's campaign feels like it is going to be a poor man's Gears of War/COD. The cover system is awkward and unfriendly to the user (moreso than ever the much derided "tank controls") and when you're fighting enemies with guns who just happen to have passing similarities to the villagers in RE4, it's given up all pretense of being a horror game.
I plan to buy the game when I have time to play it in a month or two and will probably complete Jake and Chris's games just so I can play Ada's as her story looks like it will be the best and most interesting of the game (perhaps giving closure to that lingering RE2/RE4 plotline, the way RE5 put to bed the Chris/Jill/Wesker story), but I am dreading those two campaigns given the reviews. One positive review said RE is more Michael Bay than George Romero now. To me, that is not a good thing.
All that said, I think Leon and Ada's games make it worth a purchase. But not for a while.