That's why I reckon Lionhead nailed it down by releasing demos of the DLC. At the end of the day though, it all comes down to the C, content. If the content justifies the purchase (e.g. GTA IV's DLC, Deus Ex: HR), then that's all well and good, but if it's just for new costumes, weapons, cars, etc., I reckon it's pretty pointless coz it's just cosmetic.
I can't say shit though coz I bought every single one for Arkham City.