I don't use one but a lot of my TT buddies do and swear by em. Another option is the cool-vest that you soak in an ice chest between sessions. This makes more sense to me for DE/TT days. I also sit out during July-Aug because high heat is hard on both people and cars.
FWIW, Grand Am champion Scott Pruett has been racing for 20 years and does not use a cool suit. He believes in physical fitness, hydration and ventilation to cope with on track heat even during endurance races. His co-driver Memo Rojas doesn't fare as well in the heat.
http://en.wikipedia.org/wiki/Scott_Pruett