In his five-minute TED talk (below), Gary Wolf describes the quantified self and why it matters:
The self isn't the only thing. It's not even most things. The self is just our operations center, our consciousness, our moral compass. So if we want to act more effectively in the world, we have to get to know ourselves better.
And with personal data stuff like Nike+ and Fitbit doing well, there's clearly an interest (and a market for it). At what point though does personal data become too much?