I like a tally of my miles, and I enjoy the other statistics that come with that, so I run ridewithgps on my phone most of the times I ride. I'm aiming to get stronger (and succeeding!) so I'm watching my average speed go up, so this is nice.

I invested in a nice case and mount for my handlebars, from Quad Lock. The instant speed readout isn't accurate, since it's using GPS. I could add a wheel sensor to remedy that. Another disadvantage is that it's a little tricky to operate a smart phone touchscreen while on a bike, so a dedicated bike computer would have an advantage there. But since I have the smartphone anyway, this is less expensive than a computer.