Michael Schrage, "Beta Version," Technology Marketing, May 1, 2001

Which brings us back to marketing: an inherent tension always exists between complexity and convenience. The folks at Nokia and Motorola quietly—and blushingly—confess that people only use a fraction of the programmed functionality of their cell phones. That's less true of Palms and Visors, but those PDAs represent an unusual marriage of productivity and convenience—with the accent on productivity.
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Alas, it's so much easier to sell convenience than to design it.
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Want a useful metric to evaluate convenience? Consider "Mean time-to-payback"—how long does it take someone to feel like they're getting value from a function. The faster; the better. Architecting the trade-offs between productivity, payback and convenience is what will determine whether people buy phones that are PDAs or PDAs that are phones—or neither, or both.