With the exception of ME2's quite superb Lair of the Shadow Broker, almost all of the DLC I've purchased has been unremarkable and adds very little (if anything) to the game, including the afore mentioned Mass Effect.
Releasing DLC rather than making the original game actually work as advertised, like say Empire Total War is much more of an annoyance for me though, and I hope people start to get wise to this and turn away from certain titles/developers. For instance CA won't be getting a full price purchase from me with Shogun 2, or maybe nothing at all. I certainly won't be touching any of the stuff that will presumably come out for CIV 5 either, and I'll await a proper expansion. There's almost a sense of principle about that as this game reeks of potential DLC 'additions'.