My own personal experience hasn't had any significant reactions. Most of the people I see on a day-to-day basis already knew I was considering going sly because my Alopecia I have was so noticeable. So, there wasn't quite the "shock" value that most guys experience.
However, my sister also has Alopecia, but she lost EVERYTHING (hair, eyebrows, eyelashes, etc.). We were at a water park one day, so my sister went without her wig and didn't bother stenciling on her eyebrows. As we were walking, a random lady walked up, obviously thinking my sister had cancer, gave my sister a big hug and whispered, "You are so brave. Keep fighting that damned disease!" (or something to that effect). We were all sort of caught off guard (my sister especially), so she just thanked the woman and we walked away in silence! On the one hand, my sister felt awful for allowing this woman to continue thinking she had cancer when she didn't, but on the other hand my sister wanted to spare the woman the awkwardness of finding out her assumption (and awkward hug) was wrong. To this day, we still laugh about that!