Lol I just started playing it about a week ago myself. Been wondering though if I'm missing something by not playing co-op.
It's ok single player. Co-op makes it much more fun though. Not just with randoms, but someone you level with, that's on the same quests as you, and same level, etc.
I thought BL2 was pretty enjoyable (esp playing new game + with friends), but after beating TVH mode, I'm not sure I want to shell out for DLC. Is it really worth it? I know it was $7.50 at GMG before but I passed on that.
Torgue (#2) is a good expansion in terms of story, and the equipment is good for explosive users. Hammlock (#3) is really really boring, and the new loot is horrible. Scarlett (#1) is mediocre, but you can farm gear in it like Knoxx.