At first riding seems like a good idea, but it's so much nicer to get 1/2 the mpg and be comfortable with the heater goin. I rode my bike (06' Ninja 636) almost all winter last year, granted it was a mild one. I could only stand about 30 degrees, any colder and you just shake for the whole ride to work. I always wore my riding jacket, boots, and flannel-lined jeans along with a thermal shirt, sweater, and long johns. Oh and rain sucks even with gear, some buddies of mine that didn't have a riding jacket were freezing one time we went out riding and it started raining...and it was a 60 degree day; I'd hate to see what it would be like if it were raining in 20 degree weather.

EDIT - Also factor in tire wear, sure you save gas but tires wear out so fast on bikes, is it really worth saving the gas?