How long do your users spend in the “I suck” (or “this product sucks”) zone? Once they’ve crossed the suck threshold, how long does it take before they start to feel like they kick ass?
I don’t know anybody who illustrates the motivational angle of documentation better than Kathy Sierra. Her post “Attenuation and the suck threshold” from October 2005 has one of the most insightful, funniest curve charts I’ve ever seen. It may not apply to everything you’ll ever write, but I recommend it as a fresh perspective.