There's another thread on these forums about 40 replies deep of people's experience with the DK as a multibox tank.

The general consensus seems to be they work very well, but require a little more attention then a paladin, probably less then a warrior or druid.
In particular against caster mobs, they have an advantage over the other tanks.

From what I've seen, DK's have been able to solo a lot of content that no other class can do so. My roommate was able to solo at 68, the 67 elite son-of-gruul guy in Nagrand for example.
And he's been taking quest upgrades and banking them, ie using the default level 55 starting gear because it looks so much better then the crap graphics on all the slight upgrades along the way.
If he reaches a point where he cannot take stuff, he'll equip his full set of better gear.
IMO, they are a little over-powered at the moment, but come Arena (Season 5) I'd imagine they'll be nerfed down a bit. Looking at the wow forums, the majority of the player base seems to share this opinion, although many do not as well.
Level one up as fast as you can, to make the leveling process less painful while you are in the current form, so if they do get nerfed you at least leveled a lot faster.