Instruct: Classify Amazon reviews into positive or negative sentiment
Query: What a Life saver!

I purchased these with the back wrap for chronic back pain I was having with the intent of having something that would stay warm and would be portable so I could go out into the cold without my back getting chilled. It gives out the perfect temperature to keep on long term - in fact I've used this overnight with the back wrap with no problem. It's not too hot that you would burn yourself for extended periods. I also use the wrap with heaters for overnight castor oil (packs). My first use the pack was still heating full throttle at about 40 hrs. which I guess is NOT the norm, but nevertheless pretty neat.