I think it's pretty good. Pretty very good. Niko is probably the best protagonist in a GTA thus far. Not that he has much competition. I don't think they developed him in a thoughtful manner, though. He just kind of got off the boat, looking forward to a peaceful life...then for some reason was capping drug dealers for Jacob. I think they should've made his descent from being a normal guy to being a stone cold killer more progressive, rather than immediate.
The characters who appear in the game are really great. I love how Brucie pretty much talks like his caps lock is on in his mind. The entire McCreary family is interesting. I can't understand a word Jacob says, but he's still one of my favorite characters. And everyone else is unique in their own way.
The missions are pretty hit-or-miss. The bank robbery with the McCreary bros. was one of the coolest sequences I've played in recent years, and was alot like Gears of War in how you have to inch your way through a warzone where you're totally surrounded. Other than that, though, pretty much everything just blends together. GTA4 has a problem with balancing scripted vs. unscripted events. Its scripted events are epic, but the opportunity for unscripted events that are meaningful or impressive are few and far between (Half Life 2 had a similar problem, but reversed).
Graphically, I'm in no way impressed by the game. Yeah, when you get DA CHOPPA!!! it is really neat to see the town lit up at night. And whizzing by buildings in a well-detailed car is great. Unfortunately, when you're not viewing things from a distance or watching it whizz by you...it's not that good looking. Textures are mediocre, the color palette is thin, so it looks like everything was done in crayon, and a long list of other issues. I also wish they had more cars. There are many...but not nearly enough. Voice acting, as said, was great. Personally, I wish that Niko would've talked in Russian or Bosnian or whatever the hell language people from his homecountry speak from time to time. I also don't think they should've been swearing much. I can't help but giggle every time I hear him talk about "beeg amereekan teetees" and threaten to "tare your fukkeen hart ow!" with his accent.
Online play is fun, depending on what you're playing. Team deathmatch is the best, and some other modes are fun. The setup is painfully ineffective, though.
Other notes:
-Good job making every vehicle handle differently.
-Good job with the taxis and subway system.
-Bad job with the "going out" missions, making me unable to meet somebody somewhere.
-Bad job with lack of sex scenes.
-Running still feels slow.
-Needs a better melee system.