I received a sample of these a little while ago and thought they sounded like quite a good idea. I personally would not waste my time and money on them though.
Firstly, they are not hot enough! Luke warm best describes the temperature I experienced and that is not going to relieve cramps.
Secondly, they were not sticky enough and kept falling off which was annoying.
I had such high hopes for these but they did not live up to their promises and I will not be purchasing them (I tried them through a free sample). I found that they felt weird and uncomfortable on my skin and didn't really heat up to a temperature that did anything to relieve pain/discomfort.
They are also quite expensive for something that you might be using quite a lot.
Tips: Reading through the other reviews, it seems that they did actually work for a lot of other people, so I would probably buy them once and give them a go as everyone's body is different and they may just work for you.