Just back from using this everyday for two weeks on a cruise. It was 35-40 degrees every day and bright sunshine. This product worked very well and I'm very impressed.
This was something I decided to try because a) I don't have the hair to protect my scalp and b) because I burn very easily. Yes I can just use ordinary sun protection and that works but it's not easy to apply and it's definitely not invisible: this is both of those things.
One slight(!) issue is that it damaged (only cosmetically) the inside of my cycling helmet but I think that as simply the combination of very high temperatures and the reality of having chemicals on your head under a helmet.