Ive only tried unholy, but I have to say that right when I jumped in to outlands and started questing, I realised the occasional bandage after a huge aggro pull was all the downtime I really needed. Heart strike is awsome, and my pet both holds one or two of the mobs aggro, and does a suprising amount of damage. I will be taking on four mobs, my ghoul will be on one, ill put pestilence up, and my ghoul will finish it's mob off before im done with the other 3.
I have to say Ive never played a character before where I felt so invincible in pve. I can speak for Unholy having very little downtime so far. If all im taking on is 1 or 2 mobs, then there is NO downtime. If I do pull a huge group, the gargoyle is my best friend. Questing has never been faster.
All this being said, all three specs have people that claim theres is the best, I'm guessing it all depends on how you play it. The death knight trees were designed to be versatile.
Edited, Nov 18th 2008 1:03pm by bulbonius