I played BM in pvp all through Cata, but I'm leveling an alt 85 hunter as MM and really enjoying it. It just needs to be buffed.
But with Thrill of the Hunt I enjoy having to follow my TOTH procs and their duration and deciding when to arcane shot, when to chimera shot, when to steady shot and when to throw in a reduced focus multi-shot.
With BM it seems like just a bunch of arcane shot spam by comparison. MM feels like a spec with some actual thought behind it, BM just feels like mashing buttons.
For PVP, once BM nerfs take effect, I think concussive barrage will be more important now that so many classes can dodge our concussive shots. It's nice to have a backup snare to rely upon.
So it all seems there for MM to be a fun an interesting spec, it's just flat out too weak. They ARE going to buff it aren't they? The damage parses I've seen in some fights for MM in pve seem downright awful.