Perhaps the most important item for me and my running is my Garmie. Ahem, Garmin Forerunner 305.
It works off of GPS so you turn it on and go. No need to calibrate at all.
The big display makes it easy to read with a glance.
The "Lap" button is awesome. I can 'lap' every mile (or whatever distance I choose) to see my time splits and pace per mile.
The mile alarm feature is perfect for telling me when to press the lap button. No need to constantly look at my watch, just listen for the beeps and press lap!
When you plug it into your computer you get to see a little map of your route!
There is a tab (on the computer) to see your results and splits. Makes it easy to see progress.
It comes with a heart rate strap so you can also see your heart rate and calories burned. I don't use it but it's good to have if I want to.
Most of all it is super easy to use and that is always an important factor!
I have a forerunner, and I love it! It helps motivate me to run.