I've found that an HRM can be useful on longer rides so that I don't go too hard out of the gate. I have a habit after the first 10 - 15 files to really start pounding and if I'm doing a century, it makes the final 20 miles a suffer-fest. Yesterday I did a 62 mile organized ride and averaged 79%, with a high of 92%. There were a few times in the first half of the ride that I found myself maintaining 85 - 90% and would back off a bit. I thought it helped me to finish the ride stronger.

As far as cadence is concerned, I have cadence on one of my bikes. I always felt that my cadence was in the 80 - 90 range and the computer confirmed it. I don't miss having cadence on my second bike and rarely look at it on my first anymore. Having said that, it was helpful for me to confirm that I was in the right range.