We now carry Garmin's Bicycle GPS units at B&L Bicycles.
These are popular for a number of reasons. They reduce the clutter on your bike. Most bike computers need to have a magnet on a wheel, a sensor that reads the magnet and then transmits that information to the computer on the handlebar. This usually involves wires of some sort that can potentially be broken. GPS units get their information from satellites and have no need for attaching sensors and magnets to the bike. (Some GPS units do come with these however to give a person slightly more accurate data.)
The GPS units that have mapping capabilities can be quite useful as well. You can do a new ride and then share the route information with a friend. They can either download the route to their GPS of if they lack a GPS, one can simply upload the ride information to a website and then the friend can print out the ride information. Secondly, one can use a computer to create a route and then download that information to the GPS, which will then give turn by turn directions while out doing the ride.
The Edge 800 has a touchscreen color display. It tracks time, distance, speed, ascent/descent and has the option to track your cadence, heart rate and power. (Power meters are quite expensive however). The 800 has a built in basemap and has the option to install detailed road and topographic maps. These can be used on multiple bikes without having to worry about magnets and wires. You can upload you ride data for detailed anaylisis and on-line sharing. The Edge 500 has most of the the same features but lacks the built in screen map and the ability to give you turn by turn directions. Garmin also makes the Edge 200 which is a much more basic bicycle GPS as it only computes, speed, distance and time, the basics and is very easy to set up.