I still haven't actually *played* with a Death Knight since release, but I just *created* a second one, and entered the game with him to accumulate rest xp. My first Death Knight was a Tauren, on the server where my level 70+ characters are, and where my guild is playing. The second is a Gnome, on the server where my wife is playing. And at some point I'll have to decide which one of the two I want to actually play.
The advantage of the Tauren would be that I could send him all sorts of twinking stuff, and potentially play with my guild. But as I'm not planning to make the Death Knight my main, it could be that I never get around to play with guild mates. The reason why I made the Gnome is that I have done Outlands with Horde characters three times, and now will Northrend with three Horde characters. I never did Outlands or Northrend with an Alliance character, my highest Alliance is level 60. Right now I'm tending towards the Gnome, but I'm not sure yet. Well, I first play the other characters anywyay, so I still have time to decide.