I like this definitions for ROI, by John Maeda, associate director of research and Allen Professor of Media Arts & Sciences at the MIT Media Lab:
From iThinking About iPods:
Designers can tackle the time challenge in two different ways: by making the wait shorter or by making the wait more enjoyable. In either case, the solution must boost the instant gratification by making the task more instantaneous, on the one hand, and delivering “gratification” a thousand times the value of the time used. I like to think of this in terms of ROI: return on interaction.
[Via Small Surfaces]