I've had a few interesting reactions, people who thought they knew how I did it until the last moment - it's fun pulling the rug out from under them. I don't get too many freaked-out people to the point that they run away, but then again, I live in New York...
I had my two favorite trick reactions happen in the same day at the hospital, I wrote about them in the letter E published on their blog. One little boy, after I performed a simple set and left him a deck of cards, told me I made him feel better. Shortly after that, I performed a few tricks for a teenage girl while her mom watched, and she said I was able to do something that no one in the hospital or her family was able to accomplish in the past three days - I made her smile.
That alone makes it worthwhile.