1. Purchase on Steam, tear through game, co-oping with friend, fun, fun, fun.
2. Purchase first DLC, friend's machine now crashes continually, eh?
3. Discover that while original has no DRM, the DLC stealth installed SecuROM. Great. Friend's machine has no CD-ROM (and why not these days), as near as we can figure this is cause of crashes.
4. Uninstalled, goodbye, and f**k you Gearbox/Steam/whoever's fault it was, we couldn't be arsed getting a refund but will forever blame you and won't be buying any more of your sh*t.