Alot depends on what you want to do with it, how long you want it to work, and what access you have to coolers/refridgerators. I looked into alot of different products to help me ride my bike more in the heat.

What I found was that the vests really capable of keeping up with the level of heat I was producing internally for more than a few minutes were the ones with blue ice packs in pockets and they were way too heavy and bulky and couldn't be replenished easily on the road unless I had somebody carrying a cooler full of the replacement packs.

The lighter weight vests just couldn't keep me cool very long and once they were spent on cooling ability, they trapped heat against my body and made the heat worse.

So if you are doing really heavy physical activity, none of them will likely work very well for very long unless you have access to a fridge to keep extra cold packs in. If you are just wanting something so you can be in a warm place without dying of heat, but won't be doing much activity, the vests with cool packs will work well, as will the ones that use evaporative cooling.

What I do on the bike when I know it will be hot is just always carry an extra bottle of plain water to dump over my head periodically to keep it cool. The wind blowing across my head wicks alot of water out of my hair even under my helmet and takes alot of heat with it.

One option for cooling yourself that is better than a plain bandanna and only requires access to water to activate are the Cool Cloths from Heyltex (http://www.heyltex.com/32degrees.php). I got some samples from them and plan to do a bulk buy to sell as a fundraiser for the MS150 next year.

Like I said above, they can't keep up with really heavy heat production like cycling in the summer, but they are great to use to cool down afterwards, or to keep cool when walking around or standing in the heat. The best part about them is that you don't have to stick them in a freezer or fridge...they just use water to generate the cooling.