I have been leveling my 5 DKs as blood for questing and instancing. I tried frost in PVP but got destroyed. Since then I have acquired all of the heirloom items (not trinkets) and they are better. I have also started PVPing as 5 unholy. The permanent ghoul is nice and they hit like trucks with my setup (ghetto 3-4 step). They are glass cannons, I know if I was PVPing as blood I would last longer.