I went with Mystical Skyfire Diamond in my helm, though apparently it's being nerfed (again) in 2.4. So I'll be changing that if the nerf goes through. Right now though, it has a "chance on spellcast; next spell with a cast time cut in half". Really effective gem.
I'll be going to the Chaotic Skyfire next patch when they nerf it >.<
For gems, I put +9 spell damage in every socket except my helm, I used a single purple gem there (6 stam/5 spell damage) to meet the meta requirement. I like having as much spell damage as humanly possible in PVP, because eventually we will run OOM, the only question is did all your enemies die before you're OOM?
We're far too reliant on instant cast spells in PVP with terrible mana efficiency. Blink uses a crazy amount of mana, spell pushback owns us, and it's pretty hard these days to keep ice barrier up with felhunter, arcane shots, purges and dispells. Not to mention a single warrior white crit will eat through that shield with a long cooldown anyways, and even throwing it back up costs alot.
So yeah...I gem for damage. It works pretty good for me. My lock is gemmed totally different, defensive stats mostly, but my mage is gemmed for pure offensive dps.