Commuting back and forth to work certainly does kick it up a notch - I find that it forces me to ride every weekday, where before I would slack off during the week, make excuses and also have lots of legitimate things taking my time weekday nights after I got home from work. Commuting means that the workout is built-in to my day, plus it's very convenient to tack on add'l mileage after work. The trip by bike takes as long as by train (it's only 8 mi RT). So the time "loss" in the evenings is not even noticeable. I still have time to get in a shower before my boyfriend comes home hungry!
I've been commuting about 2 weeks. I can already tell it's improved my fitness and speed!