Nightblades

Just curious, how do you guys feel about them? Strong class? Weak class? Im thinking of rolling one...

Personally, I dig them; however, I'm only a bit over level 150 with mine right now. They're fairly versatile, so I've noticed at least.

How are their tanking abilities? Can they keep up with a knight in terms of tanking?

Nothing keeps up with a knight for tanking, however nightblades are a wicked solo class, and can tank quite well in a group.  I haven't played one myself, but I have grouped with a couple from 150s right up to legend, and their solo/tank skills were pretty strong throughout all that.  They're a tank class that can do good damage with backstab and hand to hand, as well as keep themselves healed in combat and the mob focused on them with cauldron of shadows.  On top of that, they have tons of group buffs which are useful.  The only thing they lack is a rescue-type skill... but the nofail blind on cauldron of shadows makes up for that.  All in all, a strong tank/solo class, from all my experience of knowing a couple and grouping with them alot.

excellent ac with strong dodge and parry skills, moderate mob control with cauldron of shadows.  you have to chose though between starting the fight with that or backstab.  backstab at lvl 300+ has higher crit chance than nja so it's a hard choice between the two.  if you have a blinder behind the nbl it's nice but if the mob switches before you can blind you're pretty much screwed.

Bottom line for me, the more levels I gain on mine the more I am falling in love with the Nightblade class.  Like Dalk said, however, nothing compares to a knight tanking but I've always preferred a rogue type of play.