No one should buy a cooler kit. It's easy to make one, with all the correct fittings, that doesn't leak, that is as big or small as you'd like it (whether high capacity so you don't have to refill often or a specific cooler footprint to fit your car setup). Anyone can follow the directions previously posted here and have a very high-functioning cooler kit (including external lines and insulation) for <$45.
The FAST/CoolShirt shirts, however, are pretty slick. Their shirts, unlike the less expensive shirt listed a couple posts above and any shirt you're likely to build, use more cooling tubing, have two cooling circuits each front & back, have cooling lines above the chest and over the shoulders, are unlikely to kink in use and will wear just about like a normal t-shirt.
I fabbed up two different designs of a cool shirt... also with the use of a 'fancy' sewing maching with 'fancy' stitching and the shirts were unwieldy to get into/out of, required a ton of adjusting once on the wearer and the tubing was prone to kinking - I tried 3 different kinds of tubing - different sizes, different wall thickness. The shirts definitely would get you icy cold, but were such a pain the ass to use and/or would kink and a circuit(s) would stop flowing.
As many others who have gone the DIY route have learned... it's worth buying the shirt... it works very well in all regards and isn't that expensive; the cooler & plumbing is stupid simple to fab, will work as well if not better that the retail version and is considerably less expensive.
Despite poo-pooing a DIY shirt, I found it can work quite well if you're not in a racing seat and/or getting in/out of the shirt with any regularity. As such, I'm using one of my DIY shirts with a DIY walkaround backpack cooler. I'm taking the rig to Capitol Offense/Summit Point and Judge Phil - assuming the weather is compliant and the system holds up - will be wearing it all weekend. Interestingly, the Honorable Judge Phil said he was only interested if it's totally hooptie-style.
Hooptie it will be my friends!
Speedycop/NSF Racing /Pinewood Dirtbags
'10 Summit, CMP3, Autobahn, '11 CMP1, NJMP, CMP2, Summit, G'man, Stafford, Charlotte, Autobahn, ECR '12 CMP1, NJMP, G'man, NHMS1, Summit, CMP2, NHMS2, ECR, '13 CMP1, ECR, Summit, NJMP, THill, CMP2, MSR, NHMS, Sears '14 Barber, Sears1, ECR, CMP1, NJMP1, BWillow, Sebring, CMP2, THill, Sears2, '15 Sears1, Barber, Ridge, THill, '16 Sears1