I have a doppler and have been able to find baby's heartbeat from 10/11 weeks. It was really easy to locate. I just watched some youtube "how to" videos. Once you have heard it, its really easy to distinguish between baby's heartbeat and the placenta, they are totally different speeds/sounds.
I got mine for reassurance in early pregnancy and I use it about once a week.
Some people are negative towards them for giving "false reassurance" like Flisspaps says. I think that is more when you can feel baby moving, and there has been a change in movement patterns. If there ever is a change in movement patterns you need to contact your midwife, irrespective of if you can find a heartbeat on the doppler. As they will do more extensive monitoring to work out if baby is distressed.
Provided you follow that piece of advice I think they are safe and a great thing to have for piece of mind.
I bought mine for 40 pounds off ebay and that included batteries and gel.
I also agree that they're worth the money. I've never struggled to find a heartbeat, so I think Flisspaps is being a bit negative...
But yes - don't rely on a doppler to reassure you all is well once you reach the point of regular movements. I'm 29wks now and haven't even looked at the thing in the last 4/5wks. But prior to this point, it's a lovely reassurance.
I'm a big fan of dopplers and have had one with 2 out of 3 pregnancies, but at this stage, I really wouldn't bother if you are wanting it for reassurance, as once you start to feel baby moveyou will need to pay attention to that and not the Doppler. Sure if you are wanting it so that your friends/relatives can listen to the hb then fine (although there is an iPhone app that does the same thing apparently) but if you are wanting to keep a check on babies wellbeing then it's a lot of cash to spend when movements will tell you much more
I saw a consultant at 20 weeks and she couldn't find a heartbeat with a Doppler, she had to turn the scanner on ... I did panic as a couple of weeks before my midwife found the heartbeat easily with a Doppler. Thankfully all was ok. Personally I wouldn't get one. Soon you will be able to feel the baby move and that will be reassuring. Save the money for your baby.