That really depends on your speed. Like dhenry says, if you're going at a moderate pace, a 5 mile commute ends up being about 30 minutes door to door, and you don't really work up a sweat, definitely not in the winter, maybe sometimes in the summer. If you're going full-tilt you can get that down to 20 minutes door-to-door and then you sweat and you have to take a shower. So really it's a question of if you're trying to get some aerobic exercise during your commute or not.

I am of course speaking for Seattle where it's usually cool but not cold and damp but not really raining.