As the subject says. The game itself is great fun and has lots of content and things to do. Top marks for that. However, it's let down severely by the design of the PC port and the restrictions and requirements placed on it.
First, it uses DRM and you have to activate your game after installation before you're allowed to play it. That's fine while Rockstar are still in business, what about when they're not? If you can't activate, you can't play, so it's only a matter of time before your game is worthless (as RockStar won't last forever).
Second, the game initially tells you that it requires you to sign up for the Rockstar Social Club, but after it's installed and patched, it tells you that Rockstar Social Club is no longer required. Even so, it still asks you to login to the RockStar Social Club every time you launch the game even though logging in isn't required. Duh? This is just such a lazy design. I resented having to signup to the Social Club in the first place but then to be told afterwards that I didn't need to is just... frustratingly annoying. But then RockStar are good at being frustratingly annoying, read on.
Third, it requires Games for Windows Live so you have to install that too. Even though saved games can be stored locally and even though it's not required that you use a public Games for Windows Live address, you are still required to run the Games for Windows Live client in order to play GTA4.
That is just such a bad design. I don't mind being given a choice of whether to install Games for Windows Live and reap any benefits it offers if I choose to install it, but I don't like being forced to install something I have no interest in ever using.
Forth, the game uses SecuRom and so that's something else that you have to install. It feels that RockStar have put more effort into protecting thei game from being stolen than helping players to enjoy the game, and that makes you feel that RockStar have priority issues. I paid for the game legally, but with all the restrictions I can't help thinking it would have been easier to download a pirated version.
There are lots of graphical corruptions in this game. Nothing too offputting but it's obviously not a great port and with the rest of the restrictions above it just makes you feel like the PC version has been ignored and more effort has been put into the console versions. The game does play like a console game with the same sorts of controls and there's even a joypad controller shown in the options screen.
But still, if you can forgive these issues then GTA4 is fun to play and has lots of entertainment value. Some might feel the gameplay to be boring (driving around and killing people) but that could be said of any game. GTA4 is a game and any game is going to be boring after you have played it a lot.
The game requires a ton of HD space (around 16GB for the game and that doesn't even include the expansion that you get with this purchase) and needs the DVD in the drive to be able to play (another restriction, how many can RockStar possibly put into one game?!)
In the end though, if you can forgive RockStar for being so obsessed with their own profits and ruining this game's PC port by burdening it with so many restrictions, you can enjoy this game. And when RockStar goes bust and you can no longer activate your copy, you'll have a nice coaster for your drinks (activation is based on your PC spec, you need to activate the game again when you change specific PC components... sigh!)