The downloadable content allows you to play with a whole new roster of Terminators on ice themed levels with their own new missions to boot. You also get a bunch of new kit to lug around with you as well.
While it might not have been the most stirling iteration of Space Hulk it did win a few nods of approval for being so close to the board game. Maybe you have a hankering for some Space Wolves and will be picking this up.
Did you play the main game last year?