GTA4 + fun + longevity - lag = SR2
The story is better than the first, the cutscenes are much more polished and exciting (in most cases), and there are so many activities to do. I also love how everything was thought out. Accomplishments and unlockables are tracked, practically anything can be used as weapons, the menus and actions are very responsive, etc. You name it, this game is the best sandbox game ever.
The only bad thing so far: it tends to freeze every few hours. Kind of annoying, but even that doesn't keep me from playing. We need breaks though, right? lol, anyways, hopefully Volition can fix that with a patch.