We bought one when I was 10 weeks pg from E bay, brand new "Angel Sounds" for $80 and it's brilliant!! came with the gel too. We found the heartbeat at 11weeks 2 days (my Dh is a nurse so he found it easily!) but some ppl have trouble finding the bubs heartbeat just depends..... on bubs position etc too. We never had trouble finding it.
It was very reassuring in those early weeks until I could feel bub move heaps daily then didn't use it anymore.
Just one thing, don't use it every day and when you do, as soon as you find the heartbeat then only listen for 30secs probably best no more..... they are safe if not over used i believe.

eta - you will know if it's bubs heartbeat as it's heaps faster than your own! and also listen to your pulse at the same time then you'll know if you've picked up your own HB or bubs

But some ppl dont like them because if they cant find the HB they get really concerned (of course i would too!) so theres a chance that can happen and you can stress for no reason.