I had it done several years ago. You breathe into this breathing machine and it measures you metabolism. I went because I was having difficulty losing weight. They measured that I could have 1800 calories without gaining. They also gave me paperwork and diet materials to keep. I'm sure I have them somewhere still. The entire process took about an hour.
I'm glad I had it and I believe the test were accurate howeve I was still unable to lose. I was doing WW at the time, tracking everything I ate on Fitday and the WW website. This went on for about a year without losing anything so I quit WW. I still haven't been able to lose but I think my issues are hormonal. Some day I hope to have them balanced out enough to get back to my 'real' life.
If you do decide to have it I think you will like having the results. Maybe not $125. worth but it's nice to know.