Siren was my 'go to' last game, as well, and now she is a lot different. I find the best thing to do with her at low levels is to pick up Flicker in the Cata tree and then go down the motion tree while using a 'Fox' class mod and a Fire Weapon of some sort. Apparently in the late game, she gets really easy to use. Once you get enough points to get to Reaper and Ruin, rebuild your character to the Cata tree and then expand back to motion with later points. In general, Maya is probably the most useful character in co-op but probably the weakest character overall...at least the first playthrough.
Zero is basically designed for people that like Sniper Rifles or Shotgun/Melee builds. At high levels, I think he is the best character in the game for whatever thats worth.
My current co-op experience is me using Sal and my friend using Zero. Zero isn't the biggest team player but since I draw all of the threat, he gets in with melee quite easily. Since Sal can't zoom when he is in Gunzerker mode, its also really good to have a Sniper watching you. You can also use Zero's ability to sneak in and heal me when im in 'Fight for Life' without being spotted. Zero also has a team 'Crit damage' mod thats helpful in the arenas when part of the mission is killing 25 or 50 enemies with Crit shots. Generally, Zero likes the other mods more and generally my Sal provides a lot more to Zero than the other way around but at later levels, Zero starts killing a LOT more and the two should work very well together.
