By contrast, I have several Arduino boards sitting here, and never found a good use for them. For me the appeal of the Pi lies in the combination of low energy usage, low price, networking, and comparatively high computing power. Once you get over the "woah, wasting an entire computer just for [insert menial task]" barrier, it's simply incredible what this single component enables you to cook up on a whim. The Pi allows me to make things that are not yet (or will never be) available commercially.
However, I would not say this thing is necessarily useful for all hackers or nerds. It's only useful if you have ideas for small, self-built devices that require programmable intelligence. Take note that this is also fundamentally different from the stated purpose of the Pi, as stated by its manufacturer.