I also think that it's worth playing, but it's nowhere near my favourite GTA, and GTA IV is the game I did tend to get bored with the quickest. Having said that though, the amount I played GTA SA then doubt that ANY game could live up to that.
For many people I think that was the biggest problem with GTA IV, people compared it to GTA SA and while IV wasn't a bad game in it's own right, compared to SA it was never going to be as popular as that was and still is one of the greatest game ever made.