Yeah we're advised not to use them for medical reasons. BUT if you want one just to hear the heartbeat to enjoy it, then go for it. Just dont rely on it if you are worried about baby.
I got an angelsounds one, not this exact one, but similar. I started using it about 12 weeks, loved it and it was easy to distinguish between baby's heartbeat and my own. Only put it away once the baby was moving lots, at about 20 weeks. Am 27 weeks now!