I've only bought 3 of them so far; Broken Steel, Point Lookout, and The Pitt.
Broken Steel gets an automatic 'A' just for unlocking the end of the game and raising the level cap. It added some cool enemies too, like the Ghoul Reavers, and the Supermutant Overlords. The missions are ok, but I didn't find them overly challenging.
Am enjoying Point Lookout so far, although I've only done the first mission (helping Desmond defend the mansion), and have been spending most of my time exploring and killing mutant rednecks, which is fun in and of itself.
Not started The Pitt yet, will probably wait until I've finished Point Lookout first.
wasn't that good for me, I have to say, just not my kind of thing, it felt a bit claustrophobic at times and a bit messy.
I really liked Mothership Zeta
, though, even if it wasn't very challenging as far as gameplay goes. It was a nice change of pace and location and just generally geekily cool.