I'm playing 3 boomkins with my wifes two shammies (her first multibox experiment - yay!) and it's an absolute blast. I contemplated the Bear, Tree, Moonkin combination but in the end just specced them all to Moonkin and never looked back.

They can all tank well enough, they can all heal well enough, and they can all ranged DPS well enough. The only difference between them is that my designated tank druid (my "main") carries a set of tanking kit with him and the others don't bother.

The synergy in both PvP *and* PvE is awesome, although the progress of my own 5 man druid team on AA has suffered because I just can't put this team down!

Much fun to be had with this setup - enjoy, and I look forward to reading about your progress.