I'd say the free DLC for Mass Effect 2 was quite fun, though I'm not too sure about paying 4 bucks or so for a character and one mission. But the free stuff has been great so far.
Fallout 3 had some pretty sweet DLC, for a fair price at the time I guess.
I really do prefer a expansion, and I'm not sure why we've gotten away from this, especially with consoles now having HDDs and being more like PCs every generation. Most DLC is just shit, new gun for the game, wow thanks for 4 dollars? That's a real bargain! *sarcasm*
By far the WORST DLC is Modern Warfare 2's, 3 new levels and 2 old levels from CoD4 for FIFTEEN DOLLARS!? I can get complete games for the same price that are far longer and are actually worth it, it's highway robbery. It's not even an excuse that you play them over and over, it's too expensive for this crap.
Considering there are DLC packs that add up to like 5 hours of gameplay, new gear, new well everything, and charge less, that's just batshit.