Useful.........if a person is able to set it up properly and interpret what the readings are telling you. The logging or storing of the gathered information is what can make the difference in interpreting the readings. I am currently using the Nomadio React telemetry, primarily using the temp sensor and the RX/TX battery voltage telemetry. I find the the temp sensor to be very consistant and accurate and not subject to emissivity errors of temp guns due to user technique. The React will log a Min/Max/Avg temp for the length of the race you run and this is for the entire circuit you run not just for a quick stop in the pits. This has come in handy a few times when I was contemplating tuning, motor, or gearing changes. I also have my radio set up with handle vibe alerts tied to telemetry readings. Basically, if anything bad is happening to my car during a race (i.e. Low RX/Tx voltage or high engine temp), the handle vibes and I know to pull over and find the problem rather than risk trashing the car. It has on a couple of occasions kept me from going on the track with a dead RX bat due me or my pitman leaving the RX switch on. I'm giving some consideration to the new Airtronics MT-4 since the logging looks pretty good on it.