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.