I faffed about for ages before settling on this doppler which I chose mainly because of the 3 Mhz probe and because of the heart beat rate display. I have not been disappointed! In theory these don't work until 12 weeks but I am only 11 weeks and it picked up my babies heartbeats with ease (I'm expecting triplets!). I think the price is extremely reasonable considering the cost of private scans, now I can check my bubs are okay whenever I like from the comfort of my sofa.
Having now used it I am so pleased I went for one with a display, the cheaper ones without can pick up your own heartbeat but as this one tells you the rate you can check that it's not your own you are listening to (yours is slower).
The display is back lit so nice and bright and easy to read and the built in speaker is plenty loud enough and has a volume control.