I'm currently struggling with the same question. I'm currently reforged to crit, and unbuffed - I carry 22.8% crit chance. All my gems are intellect, as that gives me the most spell power. I've been debating on reforging to haste recently to combat warriors + DKs + Enh Shamans with their seemingly endless stuns and interrupts. Mind you, if I can get away with hiding in a bush, it's lights out for anyone not aware of my presence. (Talking RBG/BG...I hate arena!)
Without any procs, my standard cast time on Arcane Blast is 1.9 seconds on the first cast. That's a painfully slow cast time if you're in a one on one battle with a closing target. However...the high crit % is super inviting on procs when I've seen it reach just shy of 40%!! When your Arcane Blast is critting for close to 100k every other cast, it's really hard to want to change your crit to haste.
Anyhow, here's my armory link at present...
http://us.battle.net/wow/en/characte...whelmed/simple
And here's a little sampling of the numbers I often see while free casting in both BG/RBGs...
http://www.youtube.com/watch?v=zkhY2...el_video_title
I may make a switch to haste late this week and will post my results.