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.
