I've been commuting by mtb for about 2 years now (11 miles/day) and absolutely love it. After 6 months of it my partner and I sold our car in favour of bikes/walking/buses and it's made a huge difference to my lifestyle. The biggest thing for me is that by the time I arrive at work I've been out on the bike in the weather (rain or shine) for about 30 minutes, while everyone else is reaching for their coffee in a desperate effort to wake up. It's a brilliant destresser as well - it's hard to worry about that report when you're concentrating on your ride. I'm fitter, healthier and happier despite facing unhappy drivers, dogs out to get me and "challenging" weather every day...