Oh careful if Blood is making you think about threat, I found Unholy to be nearly universally lower on threat than Blood.
I <3 <3 it though, as it can cause damage if you sink a point into the permanent pet (500+ DPS from the ghoul is nothing to ignore in a 10man raid), and it has an additional cooldown which can actually make a difference (neither Lichborne nor Mark of Blood are really "tank" cooldowns).
Just play with it for a bit, it's interesting. Biggest issue for me is trying to get some extra crit for Wandering Plague, since without some crit Unholy seems nearly the weakest spec on AoE (now that Unholy Blight is just single target).
-- (The Death of Rats, Terry Pratchett, Soul Music)