I was hennaed in Eygpt - lovely fine tracery, although I had it on my feet because I wasn't sure of the quality beforehand.
BUT
Sometimes the henna is darkened with another substance (googling gives me para-phenylene diamine (PPD)) which can be harmful. My niece had a bad reaction, and apparently it can cause death in extreme cases (
wikipedia). Glad I didn't know that when I had it done..