There are of course drawbacks.
You have to build in transition time at both ends of work – cleaning up / drying off and changing into work clothes when you arrive; and then when it’s time to go home, changing back into bike clothes. It gets old. Some days the simple act of undressing and dressing yet again seems like the hardest part of the whole practice.
You are solely responsible for your safe and efficient progress. You’re the motor, you’re the driver – and you’re vulnerable. You must pay attention – to cars, pedestrians, other bikes, the road surface – so say goodbye to the 40 minutes or hour on your old commute when you’d read the paper or listen to the radio or books on tape. (I do see some riders with iPod buds in their ears but they are idiots.) In the same vein, when something breaks, you have to stop and fix it. This can be particularly unpleasant during dark and cold winter months. (It is a particular disadvantage compared to cars, which today are fantastically reliable, but perhaps not as much vis-à-vis Washington’s increasingly doddery subway service.)
Some days you’re just under the weather and biking is a chore, not enjoyable at all. Actual weather, remarkably enough, is not a downside. It’s just data. I’ve heard it said that there’s no such thing as bad weather, just the wrong clothes. It’s true on a bike, where if you’re warm, nothing else matters. After a few weeks of riding you’ll gain an accurate sense for what jersey, jacket, socks or gloves are appropriate to what conditions. (Write down the temperature and what you wore to help you remember.) You’ll know you’re a pro when you realize you’ve become completely indifferent each morning to the quality of weather you’re about to set out in. It won’t take as long as you think!