I used one during my labour - I had a longish early labour, but did use it once we got to the hospital also, up till when I hopped into the pool. I just had some gas for other pain relief.
Not entirely sure what difference it made (as I had no comparison

), but it certainly didn't hurt. It does take at least an hour to get it ramped up and going to really get your endorphin level up.
I hired one from a chemist, and they did also have waterproof sticky sheets that would have been ok for a shower, but not for the bath/pool.
Any specific questions? HTH