Over my 3+ years of being on XBL i have finally completed my first game and it wasnt even on my xbox.
Using my windows phone 7 to play XBL games i completed the game GeoDefence.
I really enjoyed playing the game. Its a tower defence game where there is different maps. Each map containing a line which the objects (Enemies) zoom down and you have to position guns, lasers, cannons and whatever else you have in your inventory to disable them.
The achievements in the game where really quite good. Some of them easy to obtain, as are most xbox live, but one or two of them really hard and only achieveable when you have completed some of the hardest difficuilty levels.
For me its a most have game. I hope if you have the game, you enjoy it as much as i do.