Personally, I think they are two for two on the DLC so far. Zombie Island was just about my favorite part of the whole game and the Underdome is really fun, and extremely tough until you get tip top weapons. Also, Underdome is quite useful if you are starting a new character. It has the only vending machines that level up with you consistently all the way through the game, and you can fast travel there from the start at Fyrestone. Zombie Island was designed to be played after your first run through - if you go too far into your second and level too high before going there (missions should say "tough" or "normal" - if it says "trivial" you can be sure the loot drops will be underpowered) then you'll be disappointed with the weapon drops.
yes worth buying. I dont think zombie island wwas though. short and loot drops dont seem to be rarer like it says it would be
Well, at least they didn't try tacking on a pathetic short campaign and selling it as a full priced game like they did with ODST.