We have one. It is carried in our battalion chief's rig so that it is present at every working fire.
We have the version that monitors oxygen saturation and CO saturation.
We purchased it directly from the manufacturer. It was funded with a grant.
We like it - it is a valuable tool. We use it as a standard FF Rehab tool.
We have also occasionally used it to confirm CO toxicity in smoke inhalation patients at fires.
The only issue we've had is that in direct sunlight, the probe sometimes whites out and you get no reading. That can be remedied by wrapping the firefighter or patient's hand in a towel to get the reading.
I strongly recommend this tool for both firefighter safety/rehab and for assessing smoke inhalation patients.
A RAD 57 is a nice stand-alone option for fire departments that don't run ALS and which don't carry heart monitors. You don't need paramedics to operate it and it takes up a lot less space on the rig. You can also get a RAD 57 with a methemeglobin monitor, which is a nice hazmat option to monitor for nitrate and nitrite exposure, aniline and hydrazine toxicity, and accidental ODs with old-school cyanide kits.