It was a great game, no, it was an amazing game. In my opinion it just becomes less fun over time. Sure it was the most I ever had for about 2 weeks maybe, but it wasn't fun enough to complete again, even with different ending etc. The took the less is more approach, they took less features from SA (albeit some very fun ones) and tried to make what they had better, ie better story etc, characters, and while thats great and all, there only so many times you want to play a 30 hour story. Sure the mission structure was varied, and everything was very functional, but as others said some things just became a chore, instead of being fun. SA had a good balance, sure it was wildly over the top at times, and completely unrealistic in some aspects, but it was real where it mattered, the story was great in that too, and there was so much to do afterwards. Whereas when you complete gta4 what is there to do, go browse the internet, watch tv in the game, sure they were funny and mildly entertaining, but you have to look at the overall picture.
Overall I still think it was a fantastic game, one which I am glad I spent my money on, but in my opinion the approach they had in making the game was off in some aspects, sure its realistic, but having a character who reacts the way we would isnt exactly fun, I want a character who can move with ease. Just an example but its point stands. Consicering my experience with the GTA games, I would have to say SA is better than GTA4, but GTA4 is in no means a step back, its still a step forward, it just didn't go where alot of people thought it would, myself included.