I love mine. I use them for trail riding but wouldn't be opposed to using them on cold nights in the arena. Even without the little hand warmers tucked into them they keep my feet warmer by keeping them out of the wind and dry (rain, wet brush, etc).
They're easy to get on and off too, so it's not like you're committed to having them on for the whole winter.
I have the western ones for my trail saddle, but I haven't actually put the warmers in there before--I use them just as safety tapaderos. They are easy to put on and take off, and I'd like to try them with the hand warmers in there... I don't know how strong the warming element would be when you're wearing winter boots already, but it's worth a shot.