Depends what you're paying for. The obvious one that a few people have mentioned is Undead Nightmare, which it absolutely ideal for DLC - good enough to warrant a separate release, big enough that including it in the main game would have put back the release about six months (causing the ungrateful masses to flood the internet with tear stained posts about how the developer only cares about money etc), probably too big to have gone on the original disc anyway. Charging a bit for that doesn't seem unreasonable.
Map packs, small extra features and so forth? If the cost is very low, I don't see the harm, you're getting what you pay for and more often than not the game in question would have come with plenty to keep anyone who didn't want to pay happy enough.
Stuff like Bioshock 2 adding the stuff onto the disc and then saying that you have to pay to get all of the content on there? Yeah, that stinks just a touch.
