I'd jump into a battleground on my hunter, Where i can bounce around, kill whom i please, usually the weaker people in a group, then bounce back and let people sit there and pound on the 800 resilience healer(s), Can't really do that in the arena.
Seeing as this is a very, very bursty, and burst-reliant bracket, and how Unholy can take some time to rake up to the point where it starts doing damage, You'd probably be better off with Frost, or possibly blood, but you didn't hear the latter from me, until the expansion.
I've tryed the spec at 70, twice, two different characters. The spec 'works', i guess, As do all specs, ie; mutilate. But other specs could do better. Where unholy has somewhat sustained damage, control, etc. The pick-up damage is just rather low, and in this insanely bursty bracket, that usually won't get you far. Not to mention you don't get necrotic strike until you've left the bracket.
Personal preference, i guess. If you really like the spec, i know i do, take it and run with it, but do be sure to try Frost/blood, namely the former.