I use a cheap little cycling computer... and it works great (I have more than one of the exact same computer). Then at the end of each ride I log the data to a clipboard on a paper log/form I created. Each bicycle has it's own log. All I am really interested in is logging enough miles while the weather is decent to get my annual [mileage] goal. But I keep track of the bicycles mileage as well.

I've thought of adding an app to my iPhone. I always have it with me.