today used a dk with my 4 retri pallys.
done a couple of heroics, without issues.
ofc he have great issues keeping aggro, expecially in aoe situations. his gear is somenthing i m working on.
my spec is blood. deep blood, with 8 pts in frost and 10 pts in unholy.
asap i will do dual spec, 2nd will be frost. atm cant use dual welding, weapons skills are too low.


for better infos about specs suggest this really good guide, from tankspot:
http://www.tankspot.com/showthread.p...Knight-Tanking

excellent analysis and some interesting tips.