Not ok
Absolutely ok for the price I guess. Not as accurate or relaible as more expensive coating thickness gauges but does the job. Accuracy starts to get bad around 100µm thickness and below - results are often not within the 3% accuracy specified by the manufacturer while measuring the included calibration plates. Also, don't wear any smartwatches or fitness trackers on your wrist while using it. In my tests they seemed to interfere with accuracy of the GM998.