Been toying around with one the last few days.

They are really powerful solo and I was able to tank Ramps fairly easily at 61 with my 68 Shaman x4 group (in place of my Pally).

It's a little hard to get used to tanking with a DK instead of a Pally, as if I get careless with my Pally, I can always self-heal out of trouble, or DI or whatever.

Still, I am impressed with the DK thus far and I fully intend to replace the Pally with the DK.