Useless except for bragging purposes (I did 400W!) if the 15% claim is accurate.

15% is the difference between a pace that's not unpleasant for a couple hours and being done in 10 minutes. +/- 15% totaling 30% is the difference between a one hour time trial and four hour endurance ride.

Of course it's not accurate - it doesn't know what the local wind conditions are, your personal aerodynamics, coefficient of rolling resistance which can vary by a factor of 2 between best and worst slick tires, etc.

If you really can't afford power (used second generation Powertaps with the wired little yellow computer dropped into the $200s four years ago) heart rate is a reasonable approximation for many purposes including pacing (except on shorter intervals, maybe 5 minutes or less?) and tracking training stress.