@ rotten
Hahaha, i was at 50g respecs >,>
@ main post
Yes, mutilate is viable if you play it right.
There are pluses to assassination over sub, such as the blackjack effect (which happens even if YOU break sap) or quickening(+8% movement speed, no need for minor speed with it).. +6 hit from AB's, +5 hit to boots and you are hit capped, so you can take out simple pearl ring / BoA gear in place of more survivability.
My rogue is not quite BiS.. But the damage was nice, anyway. It isn't bang bang like sub, but there isn't so much RNG, you can hold your own after an opener.
Plus you get +50%? Improved chance for instant poison to proc.
Sub would do more damage in 3 seconds, but in 30 it would lean more towards assassination.
I have not played my rogue in awhile though, since a few days after the initial 4.01 patch.. So I couldn't vouch for any changes/nerfs, though.
Hopefully helpful
~Volsci