I'm playing as Zero and co-oping with my friend and my sister, both of whom are Sirens. Zero was really cool, and on the melee skill tree could do some ridiculous damage, but now that I'm getting towards the end game (of my first run through) it seems to be getting a lot harder to stab people and not end up getting killed, so I have respecced to a different skill tree. It's a bit less fun, but I don't die as much now.
Once I have completed the game, we're going to start anew, my friend already has dibs on the mechromancer, so I'm thinking Gunzerker or Siren when we next play.