I feel the critics have been overly harsh with this game. I just finished it, having put in about 9 hours into it, I think I did the majority of the side missions too. It's definitely not Enemy Unknown, but I enjoyed it non the less.
The story was compelling, if a bit rushed in parts, and I wish they had incorporated some of the research and building aspects of Unknown but in the end I was completely happy with my purchase. The end of the game feels very rushed, and I would have liked a bit more diversity among my squad as I tended to use the same two every mission.
Definitely worth a buy though, as long as your expectations are right.