I like GTA IV but it's a bit buggy sometimes. I found loads of glitches and overall I'm not really happy with how cars looks like when their damaged. Ofcourse, it's a huge leap from GTA SA to GTA IV but ever rode on a metro track, parked your car against the station floor and let the train hit the back of your car, you fall through the map or through the tracks on the road below and your car is almost not damaged.
Same with the Swingset glitch ( of death ): WTF ?!
( If you don't know what the Swingset glitch is, Youtube it ! )