Haven't played a title in the series since good ol' GTA 3, and while some of the improvements that blew my tiny mind are from the games in between that I missed ("Water doesn't ruthlessly murder me upon touching it now?! This changes -EVERYTHING-!"),
the game promises plenty of new things. MY favorite of which, is the ability to switch between multiple, well established characters.
Playing as three different characters makes the missions I've done that required several character switches even more interesting than the ones preceding. And all three of them have the same skill progression bars to work on, with a natural affinity for some. (One is already skilled with driving, another's strength progression bar is notably farther along than the other two.) But you have to level them all for each character, instead of your skill increasing affecting all three at once. Be ready to master Flight School from scratch thrice. Not that it isn't fun, I had a blast. ("Oh my God you can fly now!? THIS CHANGES -EVERYTHING-!")
My only gripe with the game is minor and not very noteworthy. While there are plenty of cheat codes to use, one of which grants total invincibility for five minutes, I truly wish there was a more lasting God Mode. I'd like to throw away all regard for digital human life and not stress over the countdown timer until the police can completely obliterate you. It's not a flaw in the game, it's just something I feel would make the sandbox mayhem even more fun.
I've tried little of the Online portion of the game, but the character creation process was interesting and (As far as I'm aware) quite unique.
Overall I've give this game a try if you've got a few hours to throw at it. The game would be undoubtedly more fun with a large group of friends online, so take that into consideration.