During my two years at Lehigh University, I commuted to the university from my apartment almost exclusively on the mountain bike I owned at the time. It wasn't a long commute — perhaps 20 minutes each way — but it did keep me from turning completely into a couch potato when the rest of grad school was pushing me in that direction.
It was a little daunting during the rain and snow. I found that a top layer of water resistant clothing would keep my dry enough on rainy days. A face mask to keep my nose and ears from getting frost bite, coupled with making sure never to ride over ice, enabled me to get through the snow in reasonable comfort. And never buying a parking permit gave me the motivation, since you could never find a spot on snowy and icy days anyway. :)
I did fairly little cycling since grad school until the summer of 2008. I finally took the plunge and bought a road bike. I was able to find a a few relatively level routes that I liked which were about 15 miles long, which I was able do in approximately an hour. During the summers of 2008 and 2009 I rode about 5 days a week, though of course the shorter days of winter (not to mention the very unpleasant weather combined with guaranteed wind chill) made that harderin summer.
I'm really enjoy my road bike; it took me a while to get used to the different position, but the bent-over position a road bike puts you into is a much more powerful position than the upright position of a mountain bike. The local bike shop owner informed me that once your back is below a 45 degree angle to your legs, you can start employing the muscles of your butt and lower back, which I've found to be true. It takes some getting used to — for the first month or so I'd have to periodically get off to stretch my back, which wasn't used to the prolonged exertion — but I'm going much faster than I used to be.
Since moving to Pittsburgh I haven't found any good level routes to ride (I just enjoy level better, as you go faster but are still under control), but some day I'm sure that I'll find them.