I tend to break it down by company. The majority of Bethesda's DLC has been pretty legit (horse armor not withstanding). Bioware's offerings, on the other hand, are almost always some combination of too short and too expensive.
My stance on DLC as a whole is that it's meant for people who really, really, REALLY love a particular game. These people are looking for any kind of extensions or additions they can get their hands on, so cost and longevity are complete non-factors.
If you're the sort who thinks there are plenty of games out there to buy and enjoy, DLC is just a bad investment. You can save the cost of 4-5 DLC packages, which typically yield 30-60 minutes of additional content per, and simply buy a completely new game.