Cooling Vests

I've seen cooling vests advertised too keep your dogs cool, has anyone used them? Do they work? Are there some that are better than others?
My aunt has 2 french bulldogs who can overheat in the summer, which she is very concerned about, so I was going to suggest she look into these, but thought I'd see if anyone knew how effective they were first?

[gemtwinks]

I don't know about how well the ones for dogs work, but I have one for humans and I love it. I ride motorcycle with my husband and it is very easy to overheat with proper safety equipment. I do not tolerate heat well and purchased a vest made by Silver Eagle Outfitters (they make doggie products too). My vest requires sealing inside a ziploc type bag with 22 ounces of water for a half an hour. It is lightweight and works for several hours. We were on a weeklong trip last summer with temperatures over 100 degrees F almost every day. I was sick daily until I purchased the vest. It definitely helps keep the body core cool.