Great for soothing aches without side effects. Pain relief by distraction can be very effective when nothing else works. I have found them particularly good for abdominal pain. Another nice feature is that you can fall asleep with them without having to switch them off or remember how many to take or when your next one is due.
Having looked this up, I found that studies have shown than heat blocks the pain response ... whatever the reason, they can be a nice gentle pain relief whether you opt for the newer microwavable ones or the traditional ... they work best when someone else fills them for you :-)