Boomkin is incredibly fun in BGs. You are an AOE king, with some occasional burst if you can get set up properly at range and not jumped on by melee.
Being an AOE glass cannon comes at a cost. Boomkin is notoriously unplayable in 2v2 arena. You will get trained by melee in every-single-match, unable to survive long enough to cast a single spell other than moonfire (a.k.a. no damage). You will have no enjoyment.
Thing is moonkins have to build up lots of energy to cast their stronger spells (well, the one strong spell, starsurge), but the only way to build up meaningful amounts of energy is to cast spells with upwards of 2-second cast times. In this expansion, melee will *never* let you cast one of these spells, so you will have virtually no energy to do anything useful. Your cc/peel also requires a cast time and is difficult to pull off. To make matters worse, when a moonkin gets kicked you get a unique "astral" spell lockout, which silences
all of your spells.
Perhaps the biggest issue compared to other ranged classes is you have pretty much no innate survivability or defensive tools. In 2's, once they have blown their offensive cooldowns you will be dead, hoping your partner can pull off the 1v2 afterward. Your heals are also quite useless.
3v3 is less painful when you have more allied peel and heals to keep you alive, or so I hear.
Some of the defensive PvP talent changes on the PTR looked promising but sadly I don't see any of that made it into 7.1.
I'm keeping my fingers crossed for now.