My wife swears by it. She started training for her first sprint triathalon (1/2 mile swim, 3 mile run, 12.5 mile bike) about three years ago. The training she went through was based almost entirely on the use of heart rate monitors. Since that first triathalon she has built up to several half marathons, a full marathon, an olympic tri (1 mile swim, 24 mile bike, 6 mile run), and a one day STP bike ride (seattle to portland, 200 miles). So yes, I would say she has improved her fitness.

And yes, you can probably tell I am probably tell I am proud of her (and worried, I think marathoners are insane.)
The heart rate monitors help guide your exertion when you are doing endurance events, and you can watch if you are peaking too hard or too soon. The heart rate monitors in training are used to push your exertion level to a certain level, then you dial back. It is truly personalized training rather than trying to set how fast or some other measure.
I've been very impressed with how her group uses heart rate monitors.
Lamont