I have a Suunto multisport watch with chest strap, and have used it for hiking, skiing, and biking -- you do get an interesting stream of data (this particular watch can plot HR against, for example, elevation right on the display). It's also fun looking at the rate of descent when downhill skiing (peaks out at about -750 feet per minute on a fast run). As Doug said -- I have learned to pretty accurately predict my 'output' based on feeling my body and glancing at the watch in different circumstances, though I still use the watch for the telemetry aspects.
I also added a Garmin Edge 705 Bicycling GPS (also with chest-strap monitor) to my outdoor tech toys this summer, which adds the fun of plotting your course (along with all your other data -- HR, altitude, etc) on a map at the conclusion of the ride. One thing that fascinates me is how hard you're working when you're riding technical sections on a mountain bike... on long climbs my HR will run in the high 150s to low 160s, with spikes on very steep sections to high 170s... not surprising, when you're going up, you FEEL like you're working hard. What did surprise me is that my HR runs a steady mid-150s, with much higher spikes, riding very rooty/rocky/technical stuff, even downhill... so for me, mountain biking turns out to be a very solid workout.
At any rate -- I agree with Little Ricky -- having the data and studying my playtime in telemetry actually motivates me. I think a good HR monitor that suits your needs (and has the features you'll really use -- I love the altimeter on my Suunto, and the GPS on my Garmin) is a great investment.