I've been transitioning from running to bike commuting as my main form of exercise, but due to weather & job relocation it has been an uneven transition. I like to run at least a couple of days a week just for weight bearing exercise; that's enough to hold me. Once I get finished moving, I hope to get the majority of my aerobic exercise by biking in the summer and cross country skiiing in the winter. But running is usually an easy fallback when biking and skiing aren't practical.
And yes, I prefer to combine bike commuting with aerobic exercise, due to the time savings.