I'm sort of half and half with GTAIV... I love the physics, absolutely top notch for the time, really fun to plow into a crowd of people and watch them fall over, hit the car, splatter on the bonnet, try and hold onto the handles and stuff, really satisfying if not a little sadistic, lol, also I quite enjoyed the multiplayer, even something as simple as TDM in the airport was such a blast, played it so much back in the day! HOWEVER, the game itself was rather bland, there wasn't that much to do aside from FUCKING BOWLING FUCK YOU ROMAN I DO NOT WANT TO GO BOWLING! and stuff like that... although getting drunk and fighting was fun, one punch to the head would knock you down for a while, lol. The story and the characters were sort of hit and miss, but holy shit did I love Little Jacob, even if I had NO IDEA what he was saying more than half the time, lol.
tl;dr I quite enjoyed the game, but wouldn't say it was one of the best games ever like some people have said in the past, lol. Some stuff worked, some didn't. Mix IV and V together in a new location and we'd have a perfect GTA game in my opinion.