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 this summer. I finally took the plunge and bought a road bike. I've found a few relatively level routes that I like which are about 15 miles long, which I can do in approximately an hour. I've been riding about 5 days a week, though this will get a bit harder as the days get shorter.
I'm really enjoying the road bike; it's taken 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 informs 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.