In my opinion it is pretty balanced, but people got diffrent opinions. Sometimes hordes win most, some times allaince.
Currently not many people play 3 vs 3 arena, but 2 vs 2 is active as far as I have experienced.
(I think they are working on getting more activity on the 3v3).
There are also lots of world pvp, both when you level and level 80s against each other.
I don't play horde, so I don't know how long they wait in queues for RBGs. Allaince don't have to wait long for a RBG (atleast if you play on EU peak times).
Currently horde has controlled WG for 3-5 days now, as far as I know both teams has their ups and downs in WG.
I know the twinking bracket of level 10-19 is active (atleast there are twink guilds on allaince). I can sometimes see people that are twinking asking people to join in the global chat, thats the 70-79 bracket. I am not sure if there are any other twinking bracket that are active.