An immersive experience you should not miss.
My reviews go like this, I write out good, mediocre and bad things about the game. Some of the bad might not annoy you as much as they do to me and may only be minor but some people hate low frame rates for example and I prefer to tell everyone about everything that is in the game.
Good:
Great immersive story that will take you about 30 - 40 hours to complete. Fantastic characters and dialogue both in cutscenes and on the radio stations. Brilliant soundtrack with something everyone can like. Amazing visuals and details around Liberty City. Liberty City in general. Same classic GTA humour covered sweetly under a serious tone. Cover system works. Great driving and flying. New wanted system is a lot more fun. Still loads of things to do after you've finished the game.
Mediocre:
Various texture issues like pop in and such. Friendly AI is difficult sometimes. Friends like to be taken out a little too frequently. Lock on is annoying at times. Occasionally low frame rate.
There is nothing Bad about GTA IV at all.
Overall GTA IV is a must have on either PS3 or 360. For me the game is by no means perfect but it does come very close, some technical difficulties mar the overall package but are so easily overlooked when you look at the bigger picture.