I've seen him few times. Yes, as rylius said, all knifers have only good movement, maybe timing and experience. I've often saw gollumz, smeagol, dmmh, svarox (wtf he passed me???) and even myself. Thing is most people are scared of knifers. Once they are in range they have no chance.
They panic and it's really easy to kill them. About movement, knifers know where they will find people and knows when they should attack.
BTW: i forgot Slicey!!!!!! I'd dare to say he's best of us all. He is person FLYING all over place. I know how to jump, on uptown best. I know where is every corner of map, every ledge i can catch. But he, he is.... something. Not human. He can jump on places you wouldn't even imagine to jump.
And that's where every knifer comes out. From back, using passages rarely known, striking fast and quiet. You see them when you die and your camera goes back
Fear them, since it's all you can do!