I rode the other night at-5 for a few hours with no problems, but Froze the feet. Feet are the limiting factor.
Hands and feet are by far the biggest limiting factor for almost all winter riders. I can not recommend enough getting a set of Bar Mitts/Pogies, of whatever brand for the hands. They make riding in winter enjoyable for me. I rode Monday Night Rides and just a light neoprene long fingered glove. No shifting issues at all with triggers.
On the feet I would either spend the $ to get a good winter clipped boot, or put on flats and just wear a good winter boot. They also have the versatility of being used for hiking, snowshoeing, sledding, scouting new trails...the list goes on. I assume we are talking about snow biking here. I have used and destroyed a number of over-boots on trails. They work fine in cold and snow until you need to get off the bike. They don't work at all for hike-a-bike. And if you like to get out and explore in the winter you will definitely be doing hike-a-bike at some point. I know the booties are cheap, and cheap enough to try for the $, but that is my experience. If you are only riding in a couple of inches of snow or packed trails they may work fine for you. YMMV
For the title of the thread my recommendations for Winter Mountain Biking are:
#1 Best winter boot for all-around use: Columbia Bugaboo. Get the thermal and waterproof versions. Light weight, warm, waterproof, and tough. Just an awesome winter do-anything boots. I have been beating the hell out of a pair for years now for everything from winter biking to snowshoeing to snowmobiling to spring trail building. I just can't say enough good about them. An awesome boot. Again wore these to -5 this week, with nothing but a CORP sock on and feet were toasty warm. And I have worn them when it's 50 degrees out. Very versatile boot. Something like this:
http://www.sierratradingpost.com/columbia-sportswear-bugaboot-plus-omni-heat-winter-boots-waterproof-for-men~p~4502k/?filterString=s~bugaboot-columbia%2F&colorFamily=03#2 the Wolvhammer. You can get them in clipped and unclipped versions. I lust after these. By far the leader in comfort, hikability, and warmth. Also in price. Buy to fit. Warm enough extra socks are not needed or recommended.
#3 Lake boots. About $100 less than clipped Wolvhammers, the same $ as non-clipped Wolvies. Buy them a size or more large to allow room for a couple of layers of socks. Clipped only available. A good riding boot. You won't use them for anything else.