Got this xmas morning and been on it ever since. It took me a week of solid game play just to complete the story mode (only on level 37 out of 50 with 1 of 4 people you can play) but i still had quite a few side missions to finish. This game takes the best from RPG & FPS genre and combines it perfectly and you get the chance to replay the game on playthrough 2,3,4 with your current stats, weapons etc which you finished the last playthrough.
Co-op is top, if you get stuck on a level by yourself and all you need to do is host the game and you are sure to get someone to join and help you out.
If you are after a game that lasts a long time and is a mix of RPG & FPS then buy this!