They are indeed very safe. It's the same thing doctors use at your appointments. I've never personally used one, but I wouldn't suggest trying until you're at least 12 weeks and beyond.
I rented one from babybeat. With my first i got the hb at 10 weeks and 2nd 8 weeks. They are safe same as doctors office. I had bleeding with my first so it gave me peace of mind. Good luck.
I bought mine off of eBay for about $125. First pregnancy I heard it at 10 weeks. Second pregnancy I heard at 9 weeks.
I started out renting mine and eventually bought it from storkradio. I love it! In my last pregnancy and this one, I heard it at 8 weeks. Just gives me peace of mind when I worry.
I used one and it worked great! YES THEYRE SAFE!
You can also rent one instead of buying. I think that through babybeats I paid $75 for 4 months use.
It worked for me at 10w1d. I tried to use it at 10w but couldnt find him. the next day I found him rather quickly.
They've got all sorts including ones with a digital readout. :)
BABYBEATS site:
http://www.babybeat.com/
Coupon code for %5 of monthly rental:
HP04