The directions for the vest warn about chilling the packs down too cold. If the packs are way below zero and are close to your skin, you do put yourself in danger of getting frost bite from the vest. They say for best results, wear some kind of heavy cotton shirt between your skin and the vest to prevent the possibility of frost bite. I've worn this under my suit a couple of times and it does work pretty well for a limited amount of time. I think the most I've ever gotten out of it was about 2 hours of medium level activity. After the packs melt, it just becomes excess weight you have to carry around. If you're going to be in a situation where you can change ice packs over the course of the day, it would probably work great. I was doing a marathon session in the suit without breaks, so I couldn't stop and change ice packs.
If you're going to do a stint in the suit for less than 2 hours, I'd say go ahead and buy one of these things, or make one yourself. Its better than nothing, and the price is ok. I think I paid about $35 for it; but I didn't see it in the current catalog, so maybe they discontinued this model. The only cool vests they sell now are in the $100 range.