Sorry haven't been back in a while.
I've seen lots of success so far with all 3 builds, I am currently rocking the frost build and have seen a lot of the ups and downs of Death Knight raid tanking (not as easy as tabbing then hitting a mob with Conc Blow then moving back to main target) but Icebound Fort + T7 4 piece bonus + Frost Spec on AoE pulls is 21 seconds of easy mode fun.
Unholy - As avoidance goes up Bone Armor is Scaling beautifully.
Frost - The option of finishing rotation w/ Death Strike (for non-threat sensitive fights) or w/ Obliterate (for more threat) is really nice along with the plethora of oh shit buttons.
Blood - Have not messed with blood yet, however our other DK tank swears by it and is having good success with it.
For those people with 20+ 80s in your guild please go to Obsidian Sanctuary as it is a joke and free emblems + gear we got our first kill (one shot) with 21 fresh lvl 80s from our guild *I won't pug guild runs*.