I have 8 level 50 characters.... This game has the highest replay value of any game I've ever played, just my opinion. But i can only speak for myself, and I'm kinda irrationally obsessed with it right now.
This game is so awesome that I have been playing it since release day.
Did take a 4-5 week break from it around early December or so through New Year to play FC3 and AC3. Played only occasionally during that period.
I played Zer0 exclusively solo til I reached Level 50 than co-op.
I powered leveled one Maya and fell in love with her so much I powered leveled another.
After my hiatus plus the fact the 3rd DLC came out. I started a new Maya with another Live friend who started an Axton at same time. We play these two extensively co-op together with no soloing other inventory maintenance or playing slots.
The only reason this awesome game hasn't inspired me to do the other 3 classes is just like BL where I really liked Mordecai after playing Roland a great deal is I like my Maya. I still tinker occasionally with her mods and skills when I play solo. But it is awesome enough to have me playing it so much 4-5 months later.