I bought this for my son since he spends so much time outside in the winter. It will provide him with plenty of warmth and he can power it from the USB charger that he carries for his phone and earbuds. The power button is easy to reach
Fits very well, I got a small and it's snug and comfy. It takes a while to heat up but it works well.
Lets be clear. In order for this to "work", the vest has to be "snug"to hold heat pads against you body. If too loose, you don't get benefit. That being said....
The main reason for this rating is the fact that this thing does provide heat when wearing it. Its not particularly cold right now, but my boss has been wearing it all day and loves it as shes always cold around the office, so this thing does really work.
My biggest surprise is just how well it fits! I might even go as far as to say that I look good wearing it. I was originally looking for something to help keep me warm while allowing me to use my hands and potentially get my hands and arms wet during the colder months.