I think there is a lot of truth in what Jun says. I was once attacked in the street with a knife (a very stange looking one). I didn't realise for quite a while that he actually had a knife. I don't think I would have survived without my Aikido basics (Miai and moving off centre line) - but it was very scrappy. Luckily I had only a few cuts but my T-shirt looked like James T. Kirk's do after a brawl.
P.S. some knives you can buy now are almost impossible to remove from someone's hand. As is always advised - best thing is to run.
Now a couple of stories.
The careless instructor I mentioned above, the one without a dunce cap used to tell an interesting story. He used to train in New York many years ago and one of his fellow students found a teenager in his car. He had broken in. So this guy, I believe a Sandan in Aikido and with other experience, has a confrontation with the teenager. He gets stabbed and dies. Very careless and the way the story was told he expressed exactly that sentiment before he died.
On the positive side, I do know a 6th kyu who disarmed a knife using sankyu. The guy came up behind her, figured she would be an easy target, and left the knife exposed. She got ahold of his wrist and bingo. I guess I should mention that she was a former LA Cop, but the point being what we do isn't entirely useless.
Still, a trained knife fighter is very bad news indeed. I want to be elsewhere.