I read this in high school and really loved it. It was so inspiring.
I am going to re-read it very shortly, within the next week or so; it belongs on my "reread-worthy shelf."

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Update: I re-read it in a single day, this past Sunday, On Writing holds up well against the sands of time. Although it may be a little crass for some readers (when is King not crass?), I'm sure that little nuggets of wisdom can still be gleaned.
Best advise in the book: read a lot and write a lot. Yes, that means you're going to have to turn the TV off, folks.