I bought the sonoline B. I like it because it has different settings (you can measure real time or give an average over a few beats) and you can hear the heart beat and it digitally reads if for you too. I have not had any problems .... it was between $50-$60 on ebay :)
i rent from babybeat.com and i got the one second from the top with the counter. love it! its expensive to buy though, but can detect as early as 8 weeks.
I bought one from Ebay. It's "Baby Sound" brand. It picks up the baby's heartbeat but it doesn't have a speaker. You have to wear headphones. I hooked it up to the computer speaker so the kids could hear it. Another drawback is that it has a digital screen for the heartbeat speed but as soon as you pick it up to look at it, the # disappears. So you have to have someone else looking at it if you want to know the numbers. I wouldn't really reccomend it.