What more do I have to say? It's Grand Theft Auto - one of the best games ever for free roaming gameplay.
Vice city was awesome. Then when they made San Andreas, the map of GTA just got bigger and more options like restaurants, relationships and you can even make your main character go to gym. This time with GTA IV - the map got smaller but it's also more realistic. You get to interact with more structures now.
The game play is simply awesome. Great story telling for the GTAIV and I love the 3D graphics engine for it. Sometimes, I find it a little slow though.
I think this is one of the best game ever but my concerns are that some of the missions do get repetitive. It's like kill someone, drive around and deliver missions over and over again. But then again, with the freedom that you get out of the missions, it's still pretty awesome.
Buy GTAIV! You won't regret it!