Well, I can give you some limited info.... after finally getting my brother's copy to work I had a quick go... but this was on keyboard as he had lent his joypad to someone and never got it back.... the joypad I brought the little tw@ for Christmas!!
By the way, tw@ is the name of the ingame internet cafe in GTAIV
Anyway, they seemed to handle well, very physical, with lots of body roll and movement in the few cars I tried. The big heavy American muscle cars handled as you would think, and the BMW like Sentinal was fast and heavy to turn like a big four door sports saloon.
From the quick go I had they certainly handled better than previous GTA games. It felt less arcade-y than games like NFS.