Great idea, but build quality is worse than stuff in China! Bought a new pair from Hammacher Schlemmer and was desperate for it to work in the cold of winter. The placement of heating element on my two socks is asymmetrical - left sock heater is in front of toes, right sock heater under my toe joints - totally different seam and sewing patterns as if I got two socks from totally different manufacturers! The big thick lump under toe joints is quite uncomfortable when your weight is on your toes. They advertise that it takes 4 D batteries, but what they sent me only used 1 D battery per sock for total of 2 D batteries. This means the heat output and duration is much lower. The battery contacts are total junk - the spring portion will bend sideways unless you use a tool to hold it while inserting battery. The connector to sock looks like a 9V connector, but the metal is much softer and totally bends out of shape if you misalign it when attaching battery pack. The plus side is at least it gets some heat to the toes, where I need it the most, compared to heated soles that put heat under the heel. Note that the heat is very gentle so don't expect to feel any heat unless you wear this inside slippers or shoes. In fact, it was so gentle that I didn't even notice my right sock produced no heat until I put a multimeter on it, and then felt down the sock to find a broken wire floating inside.