For pvp, boomkins severely lack bust.
At low arena ratings, anything can work.
It won't be competitive at higher ratings.

For battlegrounds, its a fun composition.
Three of anything focusing on one target is good.
There are a lot of toys which are fun.
You'll have a lot of AoE.

Feral cats x3, is an option too.

For optimal DPS of Boomkins, you need to manage Eclipse.
Well you can approximate it, with a castsequence.
3-4x Wrath and then 3-4x Starfire.
But if you ignore Eclipse, they won't be doing a ton of DPS.

I love boomkins and they're quite fun to play.
For PvE, gathering quests are easy since you can remain in flight form.
Lots of toys, treants is fun with five druids.
Starfall and Hurricane is a lot of AoE.
Typhoon on round-robin is fun to blast others off of cliffs.