I've recently been getting into the Call of Duty games. Before, for some reason, I'd always not been interested in that series. I've played through Modern Warfare 1 and 2 now and I can say my opinion of the series has changed and I am now more likely to play more of the games. 
As for achievements, I would say "Mile High Club" definitely gave me the most trouble in the first game. Finally got it though thanks to a shotgun strategy I found on YouTube. Modern Warfare 2 was an easier and also better game, in my opinion. The achievement that gave me the most trouble was "Star 69" from getting all the stars in the Special Ops.