Fourth Frontier is exceptional for its ability to detect arrhythmias. I was disappointed with mine though for the fact that it wasn’t able to provide basic functionality as a real time heart rate monitor while running. It would send heart rate data to my watch only once every 20 seconds, which would show up as a brief blip and then disappear. If you don’t actually need or want real time heart rate monitoring it could still be worth it.
I’m okay with that part of it, ECGs should be read by a doctor. The device does give real time alerts when cardiac strain is detected though, which can be extremely valuable to some people. I think for a very small subset of people this is a great product. I got mine though thinking it was a functional HRM plus added features, which it was not.