----TL,DR at the end----
If the amount of additional content seems substantial, I don't mind throwing down some extra cash for DLC. I think a few good examples of DLC that extend gameplay by a worthwhile amount are those that you will find with Fallout 3 or the recent Red Faction Guerilla. Things like extra costumes or a generic character don't interest me all that much, but I accept that there are those that enjoy that sort of thing. Regarding DLC costs, well, that is just something you will have to justify for yourself. People find some things more desirable than others would, and are willing to indulge in them with their wallets. To each his own.
Now, there are some DLC-related practices that I just despise. One, is DLC that could have reasonably been included with the game to begin with. It's that sort of crap that you find available as soon as the game is released (or shortly after). An example of this would be the "special/unique" units dlc pack for Empire: Total War (though, perhaps not the strongest example).
Above all, though, is promotional or console specific DLC. This sort of thing makes me want to vomit. I'll cite a few major examples, GTAIV, Fallout 3, and Batman Arkham Asylum. All three of these games are available on every major platform (cept little ol' Wii), yet large portions of each audience are missing out on some great content. Lost and Damned, Gay Tony, All of the Fallout 3 DLC (though I hear this may be remedied in the future), and the Joker Challenge missions. All of these are tied to one specific platform or another for no reason other than some developer/publisher's palms were greased with bribe money (probably a bit over-dramatic).
Regarding the Joker challenge missions for B:AA, this has the honor of also falling under the category of promotional reward content, right along with the Scarecrow missions for Gamestop preorders. "Buy for PS3, play as the Joker now! Oh, and if you preorder from Gamestop, play the Scarecrow missions too!". I could understand if these sorts of things eventually made it to the market for purchase by everyone, but in many situations this is just not the case ("you will miss out on this forever if you don't buy it from so and so"). Another example that took this to the extreme was Dawn of War II. There were so many different items/wargear and skins that were dependent on where you preordered from, that it would make your head spin. If you were a collector of these sorts of things, this situation was just rediculous.
----TL,DR----
Most DLC can be worth it in some form or fashion, but you just need to determine what that DLC is worth to you, cost-wise. Too much? don't pay. (EDIT: and by 'don't pay' I do not mean steal)
(The rest was just me bitching about 'evil' DLC)
