I haven't actually played DK myself, but my husband has one, and from what I have seen they are a very good PVE since they can pull enemies out of crossings. As for PVP, I think there are a lot of classes that aremuch better suited for PVP, unless you are willing to pay a lot of money to make your DK strong enough to simply overpower the opponent.
If you plan on playing both your rogue and your dk, I would suggest your rogue for a pvp build and your dk as a tank, which will come in handy later if you want to farm bosses and such.