Depends on how you define good. If you want bragging right down the pub, then maybe but if you want something that gives you information you can rely on and react to, then no.
My advice is that you read the smaller print on these devices where most makes have a disclaimer making it clear that they are not medical devices and should not be treated as such. Don't even think about any device which does not have such a disclaimer - its being sold by a scammer.
Also, stop and think about it, if they were actually any good they would be commonplace and places like this would be extolling their virtues.