Usually me and my friend, but at the end i went solo and left his character at around 45 or so.
 
 
Edit: we played splitscreen

I've found myself playing with a mate recently. He's a sniper, I'm a medic-focused solider. As I'm a much higher level than him, I get help him out a lot.

I roll Solo as Roland.

Usually just 2, me and my brother.

I play solo as a soldier.

I played a bit with some friends, usually alone though. its also hard to find people around my level to play with, although with the DLC that might've changed. On the PC version for me.

My friend and I logged like 30 hours splitscreen, with varying characters n' such, 

75% Solo
20% 2 player
5% 3 player
Never tried 4 player game.

I have played almost everything but Mad Moxxi and the multiplayer achievements alone. Since I don't have any friends who play Borderlands I have to rely on pick up groups and in my experience those never work out.

ideally 4 player game, but I usually end up with just one or two others
i only play by myself if im stuck at home and in Party chat with people

i have a friend that plays with me.

I play with one other friend. Solo is boring to me and three or four people seems crowded. 

I work alone as a Soldier :)

wow did not expect so many solo's. me and my brother play together all the time.