I used a TENs machine with dc1 and found it reasonably effective (hard to say how well it worked as have nothing to compare it to).
This time round I'm hoping for water birth (lots of availability in local hosp); my understanding of a TENs machine is that it must be put on early and kept on to be effective. If this is correct, am I right in thinking it would be a waste of money hiring one as I plan to be in the bath at home and in the birth pool in hospital as much as possible?