Few things are more important than being at the right altitude on an instrument approach. In the abstract that sounds easy, but in practice I often see pilots make mistakes in this most unforgiving aspect of instrument flight.
Most of my flying is in aircraft with moving maps, as part of either a GPS or a glass cockpit, so my observations are biased by what I see in these airplanes. Oddly, I recall fewer issues in airplanes equipped with just a VOR receiver and a DME receiver; watching a DME count down is unambiguous, resulting in fewer mistakes.
