Would help if you said what you want to use it for more specific.
Are you in a big guild that you always can get bloodlust from in whichever group, are you pvping a lot, only going to raid? Do you want to tank, since you might be asked to do just that, being a class that CAN tank.
Personally i like unholy DK a lot. It's a fun playstyle, more or less impossible to stun, have fairly good CC, and a decent amount of tools
Downsides to them, for me is:
Meeting a good pvp'er with root-abilities ad libitium,
they're slow,
have no saves from falling.
They don't have a huge nuke either, if you like that from shammy.