I used these for my first few years of riding and they did a good job in a pinch when I was out longer than expected or if the temps suddenly dropped. They were also handy enough to keep a couple packs in my jacket or somewhere on my bike. Since my fingers and toes get cold really easily, I need something below 10deg.
The minor things I didn't like were that they didn't last for long rides and they made my feet uncomfortable over long rides due to the space they take up in the boot.
Switched to a pair of powerlet/first gear heated socks for $60 about 10yrs ago and they have lasted me very well. For long distance riding in the cold, heated socks can't be beat.