Mission design in GTA4 was more repetitive than in VC or SA, no getting around that fact. But also, the driving mechanics became a lot more fun to me since 4 with also somewhat improved shooting. Those alone made me have a lot of fun with it anyway. Plus I can forgive a weaker SP campaign when I consider that they also had to create a brand new engine and a multiplayer, which they didn't have to do when making VC or SA. If Episodes hadn't improved in SP department I'd probably be saying that Rockstar lost some of their touch, but they did improve.
So yeah, I can understand someone not liking GTA4 very much, especially if they are not into multiplayer, but not to the point of actually calling it "crap". It's still a very well made game in a lot of ways.