I use HR monitor tho the strap on mine broke yesterday (I duct tape it... LOL!). I like to compare quantitively how hard/easy each workout it. Or if I'm working out too hard or not hard enough. On some days, if I'm tired (lack of sleep, etc), I know being low on the HR is OK... I'm not going to force myself to work too hard.
I also do the guessing game during a workout. Based on my perceived exertion level, I usually can tell the range of my HR... in 130s, 140s, 150s, etc, and I'm usually right!