Because you asked...
I am an example of an individual who grew up with under this exact educational philosophy and I beg to differ on the outcome most of the above commentators anticipate.
Unschooling is a set of principals and ideas about learning in general which emphasizes the individual's instinctual intellectual desire and capability over institutional time based curricula. It's in no way a new concept, with people like John Holt
and Ivan Illich
establishing most of the modern ideas in this educational arena several decades ago.
Though purely anecdotal, my own case is evidence that the method does indeed work, at least in my example, and I would argue it works quite well indeed.
I grew up without school until the 12th grade, and decided to enroll as a senior in an area High School mostly out of a desire to test my knowledge and socialization prior to venturing out to the greater world the following year. I was presented with a series of intensive placement tests and tested into the top levels of the senior class, where I completed the year and graduated at the statistical top of my small class without much trouble at all.
Since graduating a dozen years ago, I attained a roll as a senior software engineer at a major financial firm where I continue to design and implement technical solutions to complex problems which interest me. I'm also considered by some a bit of an expert in political strategy and consult a number of elected officials.
All this while declining to pursue higher education and instead learning from the experts in the fields which interest me.
I find that learning from those who do is much preferable to learning from those who decide to teach instead.
Additionally, the most crucial ability a critical thinker can have is the desire for and access to written knowledge and history.
The sad state of affairs which our educational system finds itself in is one which can obviously be improved. I would think that an open system with 100% subsidy which is open to the learner to take desired courses when they see fit would benefit society immensity.
Cost of such a system would indeed be high, but quite a bit less than dealing with the problems which a lack of self-motivated education hoist upon the systems of our limited resources. In a light improvements in our system to produce better learners could be viewed as the most cost-effective move we could make.