Knives fascinate and scare me. Most good guys I know carry a knife of some sort. Most bad guys also carry a knife. There's out there. And they're dangerous.
I've touched upon knife defense in the past. I have serious concerns with how knife defense is being taught in some martial arts. Often, the techniques are unrealistic, or the students don't take them seriously enough or both. Knife attacks aren't static events, knives are retracted, slashes are repeated etc. We also don't always know that a knife in involved until we've already been cut.
Two exercises I highly recommend as either a reality check or to test your skills and techniques to make sure you are on the right track are:
1. Wear a white t-shirt, give your opponent a red marker simulating a blade and get them to attack you. See how many red marks you end up with.
2. Train with a live blade. Obviously, care must be taken, but it sure shows you how seriously you were taking the wooden blade you were training with (or whatever you use).
I came across the following video from Paul Vunak on YouTube on knife philosophy. I feel it contains a lot of good points. One item of particular note for me is the idea of attacking the attackers hands as part of your defense. It's something I've been working on for some time and can be translated to a variety of different weapons. If they can't hold it, they can't use it.
Here's the video: