My sil has had two, on for each baby. Neither worked. Second baby was an all people running c-section. She already had the epidural in when decision was made, and they knocked her out as they had no time, and her epi wasn't working enough. She has concluded tat she is in what ever percentage there is of people where they just don't work. She had a cathoder in for both as well.
I've never had one so can't comment on them from personal experience.
I know others though who had them during labour and once it was in thought it was the best thing ever and for number two they want to request an elective natural birth with epi in, then induce.