I have a very simple policy with DLC... if I want it and it seems worth it, I buy it. If not, I don't.
I bought the Operation: Anchorage DLC for Fallout 3 and will likely buy the other 2 as well. I love that game, and it's worth the money to me.
This? Not really. I love Mirror's Edge, it's actually very underrated in my opinion and one of the top games from last year, but the story mode is more my bag, and I would rather just replay that.