Essential for me - mainly to echo what Marek and Toks say - staying in controled 'zones' for required amount of time is the best way to train, even more benefit if used in conjunction with cadence monitor and better again if used with a power meter.
I've found as I get older I can use the HRM to get the same training benefits on much less riding time (useful in winter)
In practical 'on the road' use - I never use it when racing because you need to react to what everyone else is doing rather than what your HRM is telling you its safe to do.
There's not much written research on this but I find it extremely useful for Sportives - used largely to stay within certain limits, because I know I'll fall apart well before the end if I got outside them.
So I'm pretty much using the basic functions of 'time in zones' and as a measure of how recovered I am after hard rides. I never got stuck in the 'downloading data' phase beacuase I found mself poring over so much info I didn't have time to ride the bike!
HRMs are particularly good at helping you counter the 'ageing curve.'
At this age I take until thursday to be completely recovered after a hard weekend riding. The HRM taught me this - I get on the turbo wednesday evening and it won't go anywhere nerar threshold so I get off it again. Get on the turbo Thursday and its comfortably up there and I feel great.
That's very useful as it stops me thrashing myself mercilessly like I used to when I was a puppy.
But don't become a slave to it or it'll rule your life like a nagging old fishwife. You'd be amazed at how liberating it can be after using one for ages to just leave it at home and go out on the club run free from its demanding bleeps.
Yesterday's RT was so frantic it was screaming at me to slow down after only 30 minutes. So I switched it off (something you can't do with fishwives).
That'll learn it