I run in Singapore, where it is hot and humid every day.
At first I didn't believe the marketing materials. "How can that be?" "It's probably not worth it". Until I tried it. And wow! Impressive! Once it's wet, specially if you had started with ice, it will keep you cool for a long time! Haven't tried it for a full marathon yet, but I did wet it on Saturday afternoon, and it was still cold to wear on sunday morning.