I picked one up about a month ago and have used it a few times. I'm using it only along the Gulf Coast so far, so read hot and humid in July/August.
I think it's better than not using it, but by how much I cannot say. It's NOT dramatic.
Some observations, acknowledging that I am probably still learning how best to prepare it for a ride. I am soaking it as best i can, then wringing it out and letting it stand a bit before wearing. I have also put it in the freezer for a short time.
It definitely feels wet when it is on. It doesn't wet my T shirt, but feels like the T shirt is wet. That's acceptable considering I am riding in 90+ heat, but reports I've read said it feels dry, no it doesn't.
In order to cool you airflow is needed. You have to be going over 30-35 mph for noticeable effect. Others mmv, but with my Cee Bailey screen, that's how fast I need to go.
Does it lower temps by 5-9 degrees C? Doesn't feel like that. Last weekend I compared how my chest felt with the vest vs how my legs felt with mesh and no cooling attire. My jacket is summer mesh (RevIt). There was a little difference, but it didn't seem 9degrees C (about 15 degrees F).
It does stay damp a good while, 4+hours, and so far the extra layer has not made me hotter, even when it is nearly dry.
On balance, it is a worthwhile addition, but in my experience the advertising hype is somewhat overstated. It could be that in dryer climes it could be far more effective, but I have not tried it there.
Comparing it to soaking your T shirt and or filling your helmet w water, soaking the lining, it's similarly effective, but does last a lot longer.
It was $100.