I've tried it. Not exactly the same, but same principle: it detects fluid in the explored area, so the amniotic fluid in the uterus of the pregnant mare. To have a chance to detect pregnancy, the pregnant uterus must have come over the pelvic rim, and so the test is possible and reliable after 3 months of pregnancy. The earliest moment possible for testing is 2 months of pregnancy, but that's not true every time on every mare.... a positive result is indicated by a permanent tone while you hold the probe directed on the uterus.
I've tried the bigger version with bigger probe for cows and horses (normal sized horses) on 2 mini-mares, one pregnant one not: it worked good, the result was right.
I've also tried the smaller version for pigs and sheep on 3 mini-mares and it works as good as the one with bigger probe.
anyway you can have a false positive result if the bladder (yellow) is filled. It's fluid in the same area where the pregnant uterus (pink) should be, so the probe will react as for a pregnancy.
If you have absolutly no other possibility to test the pregnancy, this little machine can be helpfull. If you have to use it on many mares it is cheaper than blood-testing... but it is not really an early test of pregnancy, so a rectal ultrasound will give you the information much more early, and of course with this little machine you have no chance to detect twins in a pregnancy.
Lil hoofbeats, if you have a US-machine with rectal probe, I think there is no benefit for you to have this rudimentary US-tester. It's quick and easy to handle, you have absolutly no risk, but I think you will have done a rectal US much earlier before testing with the little machine...
I know a breeder of miniature-donkeys with about 20 mares. He has this machine (the bigger one for cows and horses) and is very satisfied of the results. With a little bit experience of where to position the probe, he's able to detect pregnancy from the 2nd month.