I have a pair of Lake MTB winter boots that I've used for the past 6 years or so. I have a pair of Hotronics heated insoles in them for the ultra cold days. The battery easily hooks onto each boot, but is somewhat heavy. The days that I don't need it the battery comes off the boots. I went up one size to allow me to wear a heavy sock.
I also have a pair of Northwave winter boots, but they are not nearly as warm as the Lakes.