The online version of the gear spreadsheet at femaledwarf.com shows that for my SV hunter, who's right on the cusp of raiding, mastery is 2/3 as valuable as crit, 1/3 as haste, and 1/4 as hit, which matches what idiggory was thinking.
Even between breakpoints, haste speeds up both autoshots and cobra shot, which is SV's regen ability - and with aspect switching and the 4.0.6 buff to autoshot, SV can use close to its normal rotation even on the move these days.
So my cat DPS toon who just hit 85 is stacking mastery, my SV hunter is gemming for agility and reforging to hit, and my healadin is matching socket bonuses. Keeps me thinking flexibly, at least.