I'm only 10-15 missions in to the game but loving it so far. The only part of the design that has me seriously worried is the point that Kordanor pointed out about 'pulling' the aliens. The optimal approach for all missions but perhaps terror missions seems to be to advance at a glacial pace, advancing maybe one or two soldiers per turn, with the rest on overwatch. For the time being, I am still finding that fun to work through, but I can see this getting old.
Otherwise, I am having a blast. I figured it had a 50/50 chance of being worth playing through great, and a 1/4 chance of being really fantastic - I'm upping the chances of the latter to even odds at this point.
Thanks to Jake S and the Firaxis team for a great game!