I enjoyed Borderlands and certainly got my money's worth, even just doing singleplayer. But "it's always just on the cusp of greatness but never quite goes far enough" does sum it up IMHO, though I have a pretty high bar for a "great" game and it can fall quite well short to still have been well worth the money and more importantly time spent playing.
My biggest gripe is the loot: there's TONS of it and 99% is trash. May be worth hunting for a mod or something that will automatically get rid of dropped loot that is crappier than you have, just so you don't have to waste time having a look at it.
Far as the DLC goes, the Moxxi one is pants (boring and highly-repetitive arenas); Dr Ned might be good if you play it early on (I played it after the main game so was 1-shot killing just about everything); Knoxx is the best, and I haven't got around to the Claptrap one (though I will).