
I am doing the four main ones. There's one that preorder people got you can purchase but it only has the Mecromancer class and the Golden Key to open a chest in Sanctuary that gives you a big weapon. There's no achievements attached to it that I can see, so I may skip it. You can get the Season Pass and save $10 of the 4 main DLC plus the preoder one, but I just got the Captain Scarlett one for $10 to play with my friend. There have been sales recently for the B2 DLC, but I think I just missed them. I'm keeping my eyes out for more though.
The Captain Scarlett one so far is very substantial with a whole new set of areas, quests and characters. It's fun and should be around five hours of content. However, I played it with my friend for almost six hours today and we didn't finish it.