I've actually never tried a full on henna treatment mostly because I don't want to change my hair color I just want the strengthening and conditioning benefits. I noticed my finer baby hairs took color quickly and if I had stuck to it I probably would have ended up with deep red highlights in my hair. My hair was also generally shinier and frizz free.
Reminds of a beauty supply store clerk who told me I would have to straighten my hair first in order to use flexirods. Indoctrination is a beast. She tried to tell me curly wont take a curl.