As I'm aiming for ultra long endurance events (half triathlon in a year and ironman in two) I follow my HR readings like scripture. Basically I only do zone training to push my aerobic and lactate thresholds higher.
Also staying in a certain zone gives me great predictability about my blood glucose levels with refueling and hydration.
The zones are odd in different sports though.
When on my road bike my sustainable speed HR is up to 145bpm. Max is about 180ish or a bit more.
When running my sustainable is up to 155 and max is 190ish
When on my mountain bike sustainable is 125bpm and max is 160.
I have test my maxes again soon with some intervals. I'm a bit worried that my maxHR is a bit low for my age. 190 at 24. Then again I have a very low natural resting HR of 46/48(43 now with added excercise) so that might add to that as well