I have one, and I love it. What I like is that it lets me sweat profusely for an extended period of time at a comfortable temperature. Conventional saunas, at 160-170 degrees, are just too hot for my comfort. The way I adjust the vent in mine keeps the temp at around 120.
One thing I've noticed about many home FIR saunas is that the heat sources are not evenly distributed. Mine has four 163 watt ceramic heaters on the back wall that I sit only a couple inches away from, yet there are only two heaters on the front wall, with the same wattage, and they're about 20 inches away from my stomach. Combined with the glass door, the effect is that my back would heat up and my front would feel chilled ice cold.
The first thing I did was tape some of that aluminized bubble wrap radiant insulation to the inside of the door to better keep in the heat. That helped a little bit, but what really fixed the problem was adding more heaters. As it happens, the bathroom with the sauna has electrical outlets on two different circuit breakers. So, I added two 350 watt ceramic heaters in front, powered by the other circuit. The radiant heat is now much more even, and it warms up more quickly.