I did quite a lot of research into this last year, and they do come VERY expensive if you buy one, and even hiring isn't cheap ... I also started to get a bit worried about the mare being uncomfortable with the alarm being strapped around her, and had picked up stories of alarms being triggered when nothing was happening. So we ended up getting a CCTV system from a fantastically helpful company and wiring it up ourselves, and combining that with a Boots baby alarm! This only works, of course, if you are sleeping close to where the mare is going to be foaling.
Having said all that, plenty of studs do use the foaling alarms, so they obviously do work well for some!