i don't think i'll ever chance my crit+spell dmg gems for haste+spell dmg.
for haste to become a nice stat, u gotta stack a lot of it, including haste in gear itslef, not by gems only. correct me if im wrong, but apart from the brutal ring and the cloak from badges, what do u have ?
i'd rather have more spell and crit than a stat that i dont consider that imba in pvp.

haste is for long fights, where it will put up to the spell dmg u lost by putting more and more haste on ur gear.
in pvp u want dmg, not fast cast. at least, IMO.