It works pretty well. Best used in the dark, flash light in front of lens, the probe in the film chamber. Then you move some markers in the display on the mobile app and it gives you the measured time. So it takes a little bit of work to use it.
My main complaint is that the data lives on the phone, so measurements on old phones are not visible on my latest phone. My work around has been to take a screenshot of the measurements (1 screen per camera usually, depends on how many shutter speeds you measure) and keep those around in my photo app. I never contacten them, perhaps there is a better solution.
Never had a chance to compare to a real shutter speed tester. However, faster speeds (over 1/60) on mechanical shutters measure indeed slower as is expected, and electronic shutters are more accurate (also newer usually) which is also expected so I use the times to under expose on some of my cameras because the shutters are slowish.
Their film processor looks really cool, but way over my budget!