Like daveF, I find it generally helps to make me feel better, especially when it comes to congestion, which a good commute seems to help break up. Be careful spitting it out, especially on a windy day or at high speed, though!

My commute is pretty non-strenuous. 7.5 miles of pretty flat terrain, save overpasses and bridges. I figure if I'm too sick to ride that, I am probably too sick to be going to work anyway. Knock on wood, I tend only to get a head cold once or twice a year at that and haven't had a bad flu in many, many years.