It's a nice thing to have so long as you remember two things:
1. It's a toy and you need to be confident/secure enough to put it down and walk away if you don't pick anything up 2. It doesn't replace medical advice/monitoring
I have one, I bought a secondhand Angel Sounds from eBay for about £8-£9, and it's a fun little gimmick that helped DH bond with the baby before he was able to feel movements, DS (2yo) likes to listen to "the bebe" too and he gets a giggle out of it. I've tried ones belonging to friends/family and from what I've seen/heard the Angel Sounds is one of the better ones (I first picked up a heartbeat at 14w with it).
There no harm in them. Just remember that you might not always pick up the baby, especially in the early days, or it might take you a little while to find it. Also remember that if you've ever got concerns or worries, fr example lack of movement, you should reach for the phone rather than the home monitor.
I had one with dc3 but found I couldn't really find the heartbeat early on and since I started feeling movement at 14/15 weeks after that I never really bothered because I could fee her moving so listening for her heart felt a bit pointless. Having said that there was no harm in it as long as you accept that you may not hear anything early on.