My basic guide for "is it worth buying X game"
1)Must get at least 10 hours of the most enjoyable gaming experience you've had (Portal 2)
2) 20 hours of really good but not quite portal 2 (Skyrim)
3) 100+ hours of... well fuck it if you even played for 100+ hours it's been worth it regardless. (d3)
4) 1000+ hours, well if you play a game that long you're either additcted or it's obviously been worth your money despite how much you cry about it on forums. (WoW)
Personally Skyrim got me 100-150 hours of class 2 gaming so was clearly worth it. As for the DLC - I'd say I would squeeze another 50 hours out of it so would definatley say it's worth it (since it's less than half the price of the original)
Part of it comes down to if you really want to be a vampire. I HATE the vampire thing - I want to use my own skills and spells not have to turn into this clumsy ass bat thing. But there'a always dawguard to join..