I have used HR monitors to pace myself for the early miles of rides with major climbs. For example, I rode the Barlow Century (a dozen years ago) knowing I wanted to make the most of the climb up to Lolo Pass. My max HR at the time was a little under 180. I knew if I never went into "the zone" early, I could do the whole climb at 165 and not blow up. So I limited myself to 155 on the early climbs. A lot of riders left me, most for good. That was OK. They were for the most part young and I was in my 50s. I rode the entire climb to Lolo Pass with the HR glued on 165 and felt great. Had a rider easily 15 years younger staying with me until the last mile or so when he dropped off.

I ignored the monitor the rest of the ride. The late hills got tough but I still was passing a lot of riders including more than a few that dropped me hours earlier.

On long, hard rides, a monitor can be a very good tool if: you adhere to it AND it is not a race or situation where the drafting element and being at the right place at the right time rules. (We just saw a Grand Tour won by a rider going way beyond any "advisable" HR, etc. in the very early miles of a mountain stage with a summit finish. He made the break, the race leader didn't. That move decided three weeks of racing.)

Ben