I find the Borderlands solo experience wonderful, and the improved story and more fleshed out characters of BL 2 only increase that enjoyment. If anything, I always do my first play through solo, and then I fiddle with multiplayer so I can play at my own pace and appreciate the games humor and story without slowing down a more experienced players progression.
Afterwards I bring along buddies for the big challenges and sweet loot. It's a shame Yahtzee won't acknowledge that with this game it's not the quality, but his taste in games that makes him dislike it.
For me the heavy loot games are for people who like the challenge and equipment bragging rights of MMO type games, but without many of the hassles of the genre (impersonal story, monthly fees, guild drama) if that's not an acquired taste, I don't know what is.