I also have an evaporative cooling vest and I think it is a worthwhile thing to have in warm weather. As Sea Steve says its necessary to rewet it every couple hours. It also works best if you wear something over it to keep it from drying out to fast. I feel it has an advantage over the phase ice packs type of vest because you can stop and recharge multiple times a day. The disadvantage of the evaporative cooling vest is that you end up with a wet shirt, but I carry a spare shirt so it is not a problem.

Good luck
Az wingrider