I haven't decided whether to buy or rent as yet but I am definitely going to give it a go. One of the girls from my preggers yoga class had her baby 4 weeks ago and she said that the tens machine was fantastic. She was in early labour for about 3 days and she used it from the word go and she said it made a huge difference. In fact, she got to use the birthing pool too and she said she liked it but the tens machine had more of a relaxing, pain relieving effect.
I think for the best effect you have to start using it early in your labour so that the endorphins build up and so you get the best effect.