I got my first Doppler when preg with number two, off Ebay, it was pretty expensive but good though I can't remember when I first heard the heartbeat.
With my third I bought the angel Doppler, only £20 or so. I heard the heart beat from 9/10weeks. I've used it with every pregnancy since and always got the heartbeat round about the same week. This time I tried at week 10, and no problems.
It's a great item, but try not to panic if you don't hear anything. Sometimes it takes a while, sometimes the way baby is lying can you mean you won't hear anything.