Always wondered ... Is any of it worth a darn?
As with anything, some works very well and some doesn't. Ask a question about a particular product and a better answer could be available.
As a general rule, those products using batteries from cordless power tools tend to work well. Heating life is dependent on battery size, temperature level, outside temp, wind exposure, how long/often the heating element is turned on, and a few other minor details. Typically, they can last as little as an hour to all day depending on the above factors. The major downsids are the bulk and weight of the batteries, either as one large one or multiple smaller ones and the cost of both the garment(s) and batteries.
The products utilizing either standard batteries or rechargeables can run the gamut from good to bad. Like most things, you get what you pay for but it is not an ironclad rule. My use of these has been limited to footwear and I have had better successful itch insole inserts than socks. I think this is due to less flexing in the insoles but in either case I don't care for them as they are too warm. Expired or low quality chemical heat pads work better for me as they are not as warm and I can put them at the top of the boot where I can remove them easier if necessary.