Mainly for running. Prior to a run I do a 15 minute warm up on a stationary bike in/on a trainer and have it on for that; it's nice to know that my heart rate is up at least to the lower end I've set. During the run it's just to let me know when my heart rate is too low.
Are you using any analysis software with it? Zone Five has SportTracks which has a bunch of 3rd party plugins for doing anything and everything.
I train with a Garmin Edge 500 with cadence and heart rate. Upload the data to Strava for the social stuff, and to Training Peaks so my coach can look at the data and plan my next sadistic pain session.