Pants I think someone had this going a few months back.

Personally, although I understand WHY mothers may want them, I am of the opinion that it's not a good idea.

You will notice that most (if not all) babies kick the doppler when your MW uses it on you, this is because the soundwaves that the dopplers use upset baby. It's loud and echoy and there are also studies coming out more recently showing overuse of ultrasound and soundwaves on a foetus can cause side effects such as fetal distress syndrome and brain damage.

In saying that, thats just MY opinion but perhaps consider all the possibilities before you go out and spend the money on something that may very well upset your bub.