I opted for a boomkin for my 5th as well. I'm not sure it's really optimal for PVE or PVP, but I do enjoy it. As far as tanking, it can't really hold more than one mob well at all. I doubt it would be good enough in heroics (I'm not there yet), but I have a pally for that anyway. I haven't done a lot of PVP, but from what I have it seems like people love to go for the giant laser chicken first. I might consider having him as a follower in the future.