Very good game, but lacking...
Borderlands is a very good game, and I've enjoyed it both times I've played through the main campaign. Looting and selling is fun (if a bit overwhelming — almost too many choices!).
I love the look and the atmosphere, and overall the story is pretty good. The endgame felt like it came out of left field, and there's a point at which the difficulty level jumps quite a bit (not necessarily a bad thing).
My only real frustration is that once the story starts moving toward the endgame, things go by very fast. I wished that I had a little more time to spend in some of the areas later in the game — perhaps a few more side missions and such.
But the game's a hell of a lot of fun, especially once you get some of the really powerful or unique weapons (Holy crap! It shoots rockets!), and I'd definitely recommend it if your system can handle it. I've got a 3.4 GHz i7 iMac with 16GB RAM and 2GB Radeon HD 6970M, and still have some framerate issues and higher graphics settings, so be warned.
The Zombie Island of Dr. Ned was fun and creepy and, in some places, nightmarishly tough. Didn't care for Mad Moxxi's Underdome Riot — just arena combat with no story. The Secret Armory of General Knoxx is the best DLC in my opinion, if a little too silly at times. Haven't made it through Claptraps Robot Revolution yet.
I've been very much looking forward to Borderlands 2, which I just picked up through the Mac App Store to avoid having to use Steam, so pretty soon I'll be headed back to Pandora in a quest for more loot.