I do buy DLC as long as it is a substantial gameplay addition or (shamefully) if it has additional achievements tied to it. I'm a little bit OCD in that sense and games companies probably know that a large majority of gamers are obssessive with completion of the games, thus take full advantage. (As seen by the recent hike in maximum retail gamerscore and the percentage of games with DLC achievements increasing).
N+ level packs, Banjo Kazooie LOG challenges ETC are all very welcome additions to a game as they allow me to enjoy something for longer. Soul Calibur IV costume packs for example are not.
What really annoys me is circumstances like Castle Crashers, where all the content is there you're just paying for the "door" to be opened to it. I still bought it I admit which will only encourage repeat performances in the future, but at least it abides by my rule of significant gameplay addition.
To raise a different point I think DLC like Tales of Vesperia is pushing it a little too far. 6000 MSP (roughly) to basically not play the game. Things like Gold, Levels and items can all be downloaded to "save" time earning it in game. I realise Tales wasn't the first, Burnout and Skate also spring to mind, but they do it so whole heartedly it's hard not to notice.
What really concerns me is the overall popularity of downloading is, and whether the future of a gaming may be slowly approaching a digital only marketplace. With game rentals behaving like movie rentals already in operation.