My wife uses the Cool Vest to help keep cool when spending any significant amount of time outdoors in AZ (6-7 months out of the year it's "warm"). She has MS, and is sensitive to heat as it exacerbates her symptoms. The vest seems to stay at 60ish degrees for several hours, until the "solid" liquifies. You can then toss the phase change packs in the fridge, freezer, or even a bucket of ice to get them to change back.

For the most part, they work fairly well. I imagine if wrapped in a jacket, they'd do a bit better so that they'd absorb more body heat than ambient. The temperature at which they phase change isn't so cold that it's uncomfortable or causes frost bite (unlike ice).