GTA IV was a huge disappointment, in that it simply stopped being fun after a few hours of play. I've spent countless hours just roaming around in San Andreas, Vice City, and so on, after completing those games. My GTA IV disc, bought on the premiere night, is now just sitting in its case, the game not even half way done. It's beautifully made, well produced, fantastic in many ways. It's just .. boring.
Can't wait for SR2 - SR1 was fun, but not spectacular - but I'm confident that SR2 will receive more of my gaming hours than GTA IV did.