Barely warms at all.  Not enough to serve as advertised.
At the highest setting, after giving it 30 minutes to warm up I had to wrap it tightly around my forearm for several seconds to even notice any warmth at all.  It may be the best that can be done with USB, I don't know.  I don't intend to try another USB pad. But it  would not serve as advertised.
Rating: 1 stars