Borderlands is the most fun I've had in a co-op game in years. The main game for 10 bucks is a steal, while the DLC at 10 for the set is a great deal too. Sure, you COULD skip Moxxi's, but for 3 bucks you can't really go wrong. The bank is worth 3 bucks alone and if you play the arena thing once it was a cheap 2 hours. (The arena takes longer than that to finish, but if you play it only once and don't want to do it more it was still a good deal.)
The 5 limit on the one DLC doesn't really bug me. I plan to play it through to completion and then...not play it. If I get all hot and bothered I'll just grab the crack.