The lowest temp on this bike (it has a thermometer, after all) - 24 F, for 60 miles. Many trips in the 20's + 30's. I don't have heat, but use a full set of leathers and two layers of thermals.
I plan on doing a LOT LESS riding like this, during this coming winter.
The lowest temp trip I've ever made... I dunno. I was younger. One night I was coming home, 120miles or so, with a handlebar-style fairing on a Honda CB750, wearing an Air Force cold weather exposure suit. It was so cold, I had to lay on the bike, and hang my toes over the exhaust pipes. On the straightaways, I was putting my gloved hands under the tank onto the engine, and steering with body english. I will NEVER be out in temps like that, again.
The worst cold-weather trip I've made wasn't even very cold. Low 50's to mid 40's. The trip was 80 miles, I'd just purchased the bike (a Triumph Tiger), and I wasn't dressed for the weather. The slacks and light jacket I was wearing were fine on the K1200LT I'd just sold (providing the rear drive didn't fail and kill me), but were unsuitable for my new bike. I actually got home, a tad hypothermic. My wife soaked me in a hot tub, wrapped me up, and monitored my temp for hours. It was a week before I would go near my bike (no matter what anybody tells you, hypothermia is painful).