I have hired a tens machine, I was alo the same under much debate as to wheter to hire one due to some peoples reviews on them.
My auntie hired one and she said as long as you use it right from the start no matter how small the contractions are then she found it worked.
It apprently defeats the object to use start using it much later in the labour and your body hasn't released enough endorphins by this time to help with the pain. My midwife also agreed that someones they do not work for women as they have been used to late on the labour. x x