I actually has an opportunity to play with the Special Ops mode of CoD4:MW2 last night with a friend. Despite a terribly laggy connection we had a blast working our way through the Alpha and early Bravo missions. I suspect the AC-130 mission was the most fun although the player on the ground is mostly just following behind the creeping barrage of explosions.
The mode is fairly well thought out. The missions are short and tactical and really reward co-operative play (re: quite difficult as solo missions). This also stands in contrast to the main multi-player game where unless you are in a clan you tend to just get on with your own thing. While most of them seem to be re-hashes of the main game areas (maybe tweaked a little) there is also some new level designs in there as well. Hopefully next time we can get the lag sorted out, unfortunately with my partner on shifts our congruent frag times are fairly rare. Any other CoD:MW2 players out there who want to work their way through Spec Ops?