Check the thread "Question, re: GPS Devices in General . . ."

Len

I bought a Garmin 301(it's an older model and can be found discounted many places) to keep me honest about my miles and pace.  I love it.  I trail run about 80 percent of the time and with the device I can keep track of my miles even when I choose to run a course different from I had originally planned-hey isn't that what we all love about trail running!  I started out with maps and still use them to plan out a run but use the GPS to give me total mileage and pace(groan!) at the finish.

It's lost signal only a couple of time, I think both times were when there was alot of trees, but it catches up when it finds the signal again.

you can download all your run info to your pc and also "mapmyrun.com". 

so, after you've slogged your way thru 4 hours of hills and it feels like you've run 20 miles...you can look at your garmin with disbelief when it reads 12 miles