I was also wondering the same thing.  I carry a 7" blade with my when hiking, and if I was ever attacked, I wonder how effective it would really be.  I guess the best thing to do would be to attempt to stab it in the face/neck.  If I was able to get a good stab in, it would probably do quite a bit of damage, but it probably wouldn't be the easiest thing to do in the heat of the moment.