What the title says, is it viable in 5.2? was it viable in 5.1?
I haven't played my rogue since cata because sub was stupid op and unfun to play.
and if it is viable, is there a decent refresher guide i can read somewhere?
Mutilate is probably the worst non-tank spec in arena on live. It's not as bad 1v1, but it's definitely no star performer. Currently no rogue specs are desired in RBGs, and mutilate brings nothing compared to the other specs.
In 5.2 all rogues will be stronger. Mutilate should come out of it as a sustained spec with decent armor-ignore and reasonable uptime. I suspect the other two specs will continue to be stronger in pvp, but if your heart is in mutilate you should be able to do something with it in arenas in 5.2.