Garmin Forerunner 305 is like having a training partner on your wrist. Easy to use, with many features that help keep track of whatever type of workout you do. I use it on my road cycle, on a run, and on my treadmill.

The cons are its a bit large, but you do get used to it. And you need to buy a few expensive accessories to get the best benefit (i.e. shoe attachment for treadmill running, cadence box for bike)