They are indeed very clever pieces of equipment. I can't run without it. It logs everything, and you can set it to 'auto-lap' to a mile or km.
There are many features on it too advanced for me to use or need to use.
My main uses are to log distance, speed, pace, laps, elevation (the hills, lots of them in Dover!) time and calories. i then plug it into the computer when I get home and upload it. I have almost 5 years of data now so can easily compare one run with another and see how much I have improved over time.
(I am very proud to admit that I just won an award with my running club for most improved female runner of 2011
)