I hired the boots one. Worked great, 9 pound baby with no other pain relief. I have bought one this time (wish I had bought the first time as the price difference really isnt that much) I think the boots one has less bells and whistles than others, but the basic kit worked just fine for me!
Customer service was excellent for hiring. There is no real time limit. If you go over your DD just ring them and you get a free extension.
do make sure you do a trail run before labour so you and your birth partner know how it all works.
I borrowed an ancient old TENS from my midwife for DD1's birth and it was absolutely fine. No idea of the brand but it looked like something from the 1970s. For DD2 I bought a second-hand Lloyd's Pharmacy one from eBay (dead cheap) and it was also absolutely fine. The main thing is that it says it's obstetric (ie for childbirth and not just general pain relief) and has a boost button. That's all you need. They all have a dial to increase the intensity as your labour progresses.
It definitely helped for both my labours by the way.