I'm a "silver" gamer (age-wise) so this game's hip-hop, rap, gang warfare and 'gangsta' culture is a bit out of my usual line of interests as RPG player.
Many people have compared GTAIV with Saints Row 2 (SR2). I do have and enjoy GTAIV, and I am sure enough has been said on the quality and content of that game.
SR2 I agree has a similar structure but excels in irreverence, humour, lunacy and great action which combine to make it a very different game, and one I find great fun to play. Oddly, graphics vary from very good to rather PS2 level, but after I while I tend not to notice. However during the FMV movie sequences your created character remains in the looks and clothes you give him (and the weather / surroundings also maintain continuity). Many games do not achieve this and I really like that touch.
Apart from some niggles and frustrations, I find game play fun and the AI of non-playing citizens of Stilwater provide many chuckles. For instance I paused to watch a fire-eating juggler perform (a bit surprised to find this in a game) only for a passer-by to catch alight, scream, run into my character setting him on fire too. All sorts of little surprises like this pop up.
AI can also be dumb, like a shoot out at five paces with machine pistols. It takes a few thousand point-blank rounds to persuade the target he is supposed to be dead!
There are an immense range of motors from sludge lorries to motorbikes, missile-equipped helicopters, jet skis, etc. Powerboating at speed with SR2 presenting stunning views of sunset over Stilwater (graphics were spot on, there) took me right back to Miami Vice days (and the nearest I'll ever get to owning a powerboat).
I do get frustrated over driving - vehicle steering is just awkward on the PS3 and it really should be better. Maybe THQ could patch this? There are a number of glitches - finding one person half buried in a mall pavement but yelling on a mobile was weird. I also find the character creation wardrobe element buggy and prone to crashes. Save regularly. These reduce my overall star rating.
I rarely do on-line multiplay so I can't comment on that. This is not a game for youngsters - the language is strong and there is a great deal of violence. Stick to the age limit please.
I bought this because other reviewers highlighted the irreverent fun of it. They were right and I am really enjoying SR2 and even its 'gangsta' theme. I am now anticipating SR3...
UPDATE: One year on, storyline well completed. I'm still pottering around Stilwater finding new things, acquiring cars, boats and planes, customising things and my character, and having a great time. This game is a gem.