Lars Erik Holmquist defines what he means by "extreme users" in his paper: User-driven innovation in the future applications lab:
users that have very particular and perhaps peculiar requirements. We believe such specialized groups are more likely to put our technology in a new light, thus giving rise to interesting ideas. We can think of them as “extreme users”, an analogue to the concept of “extreme characters”, which are persona that are created to generate ideas in interaction design. As with extreme characters, the purpose is to inspire novel ideas that can be generalized for a larger audience. In several instances we have seen how the insights gained from working with specialized users has pushed the original technology and concepts much further than would otherwise have been the case.
Why do I blog this? I like to think in terms of extreme users and extreme "usage".