Early on I decided to log my commuting riding separately from my other (more mountain-bikey) riding.
This has been great, and I use Runkeeper for the commuting, and Strava for the other. It keeps all the commute stuff off Strava, but still lets me keep track of what's happening.
From a commuting point of view, I have averaged 29km a week over the last 4 weeks. Work is about 2km away so this equates to about 14 trips a week which is pretty good! I come home for lunch most days, so can easily do four trips a day when I take the bike to work.
This riding is completely replacing an equivalent car ride, I'm not doing any more or less than I would if I were using the car.
The IRD rate for car mileage is 77c per kilometer, so I am currently saving $22.30 a week by riding - over a $1000 per year.
But wait, there's more!
This also means that I have done about 1 hour 40 of exercise per week, which averaged over the 5 days means an average of 20 minutes a day. Which is close to the the recommended 30 minutes a day - without even trying, dressing up, or inconveniencing myself in any way.
Effectively I'm being paid to exercise.
I find myself using the bike more and more. Since I got the backpack I can go to the supermarket, and carry my laptop back and forth to work - it adds a lot of flexibility.
My ambition now is to take the bike to the airport when I'm going away just for a day. That will save $32 in parking charges on top of the kilometers!
Bike commuting is as quick as the car for the short distances I go, and a lot more enjoyable.
Don't just take my word for it though, here's a link to Mr Money Mustache on the economics of bikes.