The car handling was much better in GTAIV. Cars felt they had some weight to them, they stopped slower and skidded and the colisions were more realistic. In GTAV it feels more fake, cars turn way too easily no skidding and some cars are way too twitchy and colisions and damage don't seem right for the impact takes may more hits to damage cars.
Everything else in the game is great though. The graphics are awesome for a PS3 game, no blur and antialiased and not much noticble pop ins. Body panels can also come off the cars is some situations so in a way the car damange is improved but less realistic. If you could have the GTAIV car handling and increased damage for all impacts this would be a much better game though, don't know why they changed it so much.