Can't believe how much time Borderlands has eaten for me.  I think I probably have over 30 hours and I'm finished the main game and the Zombie Island DLC.  That is on  playthrough 1 BTW.  Don't think I love it enough to do a playthrough 2, but I'll kick one off and maybe it will grab me.  However, I took one 40 minute run with Underdome (2nd DLC) and that was enough for me.  
 
5 rounds of 5 waves each was just way too damn repetitive for me by the end of the 3rd round.  Then I happened to screw up and get killed in that 3rd boss round and the second wind didn't pan out (boss was the only enemy to kill) so I was dumped back at the beginning of round 2.  Ouch.  Did some googling to see if the DLC gets better and found out it actually gets worse.  For those who haven't played through, apparently the secondary challenges have you playing through 20 rounds of 5 waves each in each of the three arenas.  And you can't save in the middle.  And you don't get experience or loot for your kills.  Over three hours of straight tedium, three times...
For me it's actually a bit liberating as I was contemplating S-Ranking the game and all 4 DLCs, but there is no way I'm getting the Underdome achievements so that frees me from fooling with playing missions in split screen for the co-op achievements and some other things I would have to do artificially.  Don't get me wrong, I already did the co-op achievements you can pick up in a minute or two in split screen.  :-D
 
Thankfully I bought the GOTY edition of borderlands so I didn't pay $10 for just Underdome.  I could see if I had paid $10 how I would probably feel obligated to schlog through the boredom.  Also, while I am pretty motivated by achievements, I am sane enough to not beat my head against a wall playing a mode of a game I don't enjoy for 12+ hours for a few of them.   If your progress was saved after every round, I could see myself eventually working through this mode playing a couple rounds here and there.  But, frankly I very rarely play a video game for longer than 2 hours at a stretch - really 1 hour is typically my session time.  Three 3-4 hour sessions do not appeal to me.
 
Too bad they didn't make a proper expansion with a story and various missions like they did in the other DLCs.  My biggest complaint with the rest of the game is it got way too easy after my character leveled up, had some decent weapons, and enough cash to top off my ammo at every vending machine.  But there was something about it that kept me wanting to play even with it feeling a bit easy.  I think it was the variety in the story and the humor.
 
I do wish they just made the bank so it is accessible at every New-U station and added more than one New-U station in each DLC pack.  Warping close to a new objective is more fun (IMO) than fighting to clear the same sections of the same enemies every time you fire up the game.