I also have one of these and find it's distance function agrees extremely well with my Cat-Eyes as well as providing heart rate, simple navigation and many other useful functions. The only shaky data are the "total climbing" numbers which, on my particular 305, seem very optomistic.

One real advantage of the 305 and similar GPS based cyclometers is you can transfer them from bike-to-bike and never have to worry about wheel and tire calibration.