Well I purchased gta 4 saturday. Spent some hours with it. It is fun but not a 10/10. I would say 8.9. My problems with it are as follows:
Driving system/physics: controls are not very responsive especially when taking curves, so it takes a while to get used to. Have to use brakes and handbrakes alot.
Hand to Hand combat: parry/dogding and counter attacks don't work all the time.
Niko went through a couple walls, not much to talk about, but with all that time spent on the game, wasn't expecting that.
That being said. It is pretty darn fun. I was watching tv in roman's apartment b4 doing anything, it had more entertaining programs than tv in real life. The radio stations are hilarious, the story is going very good so far and its just a lot of fun hijacking cars, it never gets old.