I have the Mountain Horse Ice High's, which are a newer and less clunky model than the older Rimfrosts. They are a tall boot, really warm. They don't quite compare to a leather boot for leg contact when riding, possibly because mine aren't tight enough in the leg. Not awful, just not as nice. If it's not horribly cold I wear the MH's while I get the horse ready and then switch to my leather tall boots to ride, then switch back as soon as I get off. If it's really cold (like -20C) or I'm just hacking then I'll wear the MH's to ride as well.

“It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.”                                                                                                                           ~Mark Twain