This extended essay that gives a brief overview of the main aspects surrounding the 'free will' topic will likely only be useful to those who have not explored the topic. Harris covers the main ground (i.e., how could you act independent of prior causes?) but does not offer scientific nor philosophical depth. In this way, it was disappointing, but I take solace in the fact his work will contribute to a larger (increasingly necessary) discussion we should be having as a society.
Given that the piece can be read in under an hour, you see it as a refresh or as an introduction to other readings.
Then again, you could probably just read the parts of The Moral Landscape
where Harris talked about free will (and if you don't have the book, someone has put the audio on youtube).