We signed up on the first day by Preludes. It was relatively easy to get the time/day we wanted.
I would suggest you do a search under this subject, because it seems to come up every few weeks, and there is always lots of debate over it.
DD4 had her hair done on board, and she loved it. It was not painful, and let me tell you - she screams when I even hit a knot. However, there are lot of people who report it hurt a lot, some even give their child tylenol before.
Also, the debate over hair loss is a big one. On one of the earlier discussions, a hair stylist said it is not that the braiding causes hair loss, it's that for the 3-4 weeks that your DD has the braids, the normal hair loss is not falling out - because it's in the braids.
I don't know who to believe, but we noticed no unusual hair loss when I took DD's braids out.