I started full-time bike commuting my first workday of 2012, so every new year forward will provide a convenient milestone. I gathered a few stats from my first year, and I thought it might be fun to see various stats that others track as well; let's see what you got?
I failed to note my cyclocomuter odometer at the beginning of the year, but I know that now it's just over 5000mi, so I rode about 3000mi last year, commuting + recreational (some trails, as I can swap my computer to my mtb as well). I did however note my car's mileage, so I know that I drove it only 2062mi. Actually, a little bit of that is my wife (but not much because generally "my" car lived at work, parked full time in the parking deck) -- and that doesn't count miles I drove in "her" minivan, mostly with the whole family.
I don't know exactly how many days I commuted, I think it's probably near 200. I persevered through my excruciating pinched nerve early in the year, but I had to drive for a few weeks solid in the summer when recuperating from my MTB crash. This year I'm tracking my exact number of days over at the Bike Journal Commuter Cycling Century Challenge, my goal is 200 days, including at least 20 "Perfect Commuting Weeks" and 2 "Perfect Commuting Months".
We buy almost everything with our Costco rewards Amex, so it is easy to check online for spending in various categories, including gas. In 2013 we paid about $1500 (28%) less for gas than 2012 -- which presumably means something like using 28% less gas and driving 28% less (although not exactly because gas prices and fuel economy vary). The difference of about $1500 is for all practical purposes the same as what I spent to build up my commuter bike. So in one sense that's a wash, but at the end of the year, I still have a whole bike that will last me many years, plus better health and quality of life.