They're a novelty. You'll be impressed for about 5 minutes until you realize you can't really read all the information you'd want. The standard methods of testing are more accurate, but if you just want to fiddle fart with it, go for it. I thought I needed one years ago and bought into it but never found any great use for it that would replace standard testing. I did enjoy it, I just never did anything useful with it.
They've been around for decades but they have no competition. What does THAT say?
You cannot use it safely on load and that is exactly where it would be useful.