
I bought 1 when they first came out for a bitch who had been impossible to get in pup. It's not something you can just use as the bitch comes into season as you need to take readings to get an idea of the usual state of your girl, so forward planning needed. I used it on the bitch 3 times, and got a litter each time, but have never even considered using it again as it was such a hassle with the charts etc. I did lend it to a firend to use a few years ago, and they got a litter too.
So I would say, yes, used with care they work, but are time taking.