I bought one at around 10 weeks ( I'm 21 weeks now).
I do agree that if you use it as a 'home' piece of medical kit, then it's not really a good idea. If I had any pain/lack of movement/odd symptoms, I wouldn't rely on it to tell me anything.
But as a cheapish piece of kit that gives me a unique perspective on my 1st pregnancy, I really like it. My partner loves listening to it to.
I guess there's a danger in later pregnancy in confusing hearbeats (thinking your own is the baby's) - but early on, I don't see how anyone could mistake it. I can hear mine strolling along at around 68 bpm, and then the baby's galloping along at more than twice the speed.
It also moves around a fair bit too. Which I guess someone else might find frustrating, but before I could feel any movements, it gave me a sense of the baby as 'active', which I really liked.
So I'm glad I bought mine - but I bought it for fun and curiosity, not for anxiety/reassurance.