Full disclosure.....I'm the Hottronics rep. at Stowe. Just received my new M4plus batteries and heater pads!!
Last year I field tested the M3 and the M4 batteries for HT.
Things I liked about the new batteries:
The shape is curved a tad to match the boot.
Especially the M3 is tiny, but lasts all day.
Can re-charge at any time very much quicker than the older batteries.
I don't suffer from from cold feet to any great extent...except my heels(frost-bitten from a botched fitting while being sponsored by Salomon in France!)
However, Dobermans are cold boots, and last year was the first time I used
my heaters fairly regularly....what a difference....I could be outside for 6 or 7 hours without freezing feet.
I disagree that people do not stick with boot heaters from year-to-year, I know of no-one who has used the heaters and not been hooked.
Comprex, I have used the neoprene shells, they definitely do have an effect. The only issue I had with them is that the band of material under the boot tends to wear out quickly. But, otherwise they are quite useful.
I've also used disposable chemical packs...they work well, but are uncomfortable in properly fitted ski-boots.
Up here in the frozen North, cold feet are a subject of much discussion, many people have Rube Goldberg concoctions with multiple batteries etc...