I have a Mio C320 (?) and it's only "ok"... but I found on line that somene had come up with added features by exploiting the fact that it has a SD card slot and runs windows CE.. now I have all kinds of cool stuff on it.
I also have a Garmin Forerunner 201. It's basiacally for fitness training, but I've brought it up on the mtn with me and had it track my day. I can then take the days runs and view them on their software and actually see my speed, altitude changes and have it all plotted on a map.. pretty cool.