Ah, getbig science in action. At least in the responses so far, it looks like people who use it think it works. Which, I guess, is not shocking.
There are two schools of thought, not necessarily competing, on what they do. Some people say that their primary purpose is making you sweat around your stomach which causes you to burn belly fat faster. I am skeptical of this. This just sounds like classic spot reduction myth.
The other is that it essentially gives you an ab workout that lasts for however long you have it on. This isn't entirely unreasonable, but I guess the question just comes down to how effective that "workout" is.
Might try, but will probably wait until later in the year. Most of my summer shirts are on the tighter side.