I really like it so far. Feels like a new game (except that the city is still the same of course).
I love the referrences to Niko at some points in the game. Even in the intro, you can already see him walking on the sidewalk, bumping into some dude.
The bikes are... well... still GTAIV bikes to be honest. Handling feels maybe slightly better at best. The biggest improvement is probably that the handlebar is finally connected to the rest of the bike, and at least it looks realistic now when you are steering. Other than that, still the same old bike handling IMO.
Sometimes it looks odd how your gang is maneuvering on their bikes next to you/behind you. It can also provide some great fun however... speed up a little bit, use your handbrake to take a turn in the last moment... then look back and start laughing at your fellow bikers who desperately try to take the same turn, but fail big time, miss the corner and crash into whatever is in their way!
Haven't done much of the missions yet, but the few i did have been good fun. Overall i really like the new setting.