It has made me happy.
I spend a lot of time on my bike - every other day is "ride day" and that means I try to log about 110-140km on those days. My off days are either spent not on the bike, or biking with family, or running around doing errands (on my bike, of course.)
It has prompted a collection of bikes. I have four of them right now. One old race bike (does not get ridden at all but is a classic and I have had it since I was about 14 or so), one old mountain bike (does not get ridden much - 128km on it this year), one cyclocross bike (2464 km on it this year and climbing, rapidly), and one commuter/city bike (3079km on it and going up rapidly). I don't have enough bikes. For some reason, there is always one (or six) more that deserve a hook in the basement to hang on. For example, right now I want a Kona paddy wagon (because inexpensive fixies rock!), a Kona Major Jake (because, well, because!), an Electra Rat Fink (because it is just so cool), and a Kona Dew Delux (not because there is anything wrong with my Dew City - far from it - but because this one is just that bit lighter, and has disk brakes which I think I would really appreciate in the snow.)
My fitness is better than it has been since I was 29, a long time ago. Then, I spent hours in the gym and did five spin classes a week (two back to back often) which ripped the excess weight off of me and made me strong as all get out but did nothing for my endurance. Now? Big time endurance and I feel very fit, but have not ripped off the excess weight like I did before. It is a slower process, but one which is more sustainable and long lasting. More lifestyle related if you will.
It has proven to me that it is possible to significantly cut down on my use of vehicles powered by fossil fuels. I have been bugged for a very long time about the destruction of our environment by our habits in our culture and have decided to do something about it. Being one person, my ability to effect systemic change is limited, but I can and do make a difference when and where I can and am not shy about talking about it. Yes, I still own a car because yes, I need it for some aspects of my work (variable work locations some of which are too far away to bike to in the winter and none of which have shower locations I can use spring, fall, or summer) but no, I don't use it all the time and in fact up until the recent travel for vacation (a few out of town trips) I had almost the same km total on the car as on the bikes this calender year. A real switch and one which I hope in future years swings even further toward the bikes. Using steak powered vehicles really makes you consider how much energy is used for transport.
The most important thing though is that it has made me happy.