As a former Blood DPS death knight prior to path 4.0.1, I changed to Unholy DPS, and with recent spec opinions gave Frost a chance. With Unholy I felt middle-tier with any 10m RAID, and while I did fairly well, it never felt like I did what I could.
With Frost spec, I have noticed a drastic performance change using the ideas of rotation and talent choices provided here in this post. Frost has changed my DPS to not drop and my total damage to increase significantly. In a raid environment it can be difficult when Single Target is the case, but when you have multiple trash with Boss it is overkill and noticeable that your damage is there.
I suggest for any straight DPS players using the two, try Unholy spec and Frost Spec as OS, and run a raid, switching off between the two depending on your boss encounter and the Single/Multi target situation, you'll see what I mean.
Frost is easier I believe to play, while Unholy has more and's and if's tied to it, when you wait for cooldowns and runes to regen. Just my two cents, will keep an update of what I discover in my next ICC 10man.

