I recall reading a number of spin related articles on Fangraphs and Hardball times and the only firm takeaways I could recall is that the time from well a ball is not spinning (in the hand during the throwing motion) to the point were is has been spun is silly small (fractions of a second) so a pitchers spin tends to be very much tied to their specific body type/throwing style. Pitchers with higher release points tend to impart more spin. Spin rate tends to scale linearly with FB velocity. Pitchers avg spin rate is pretty consistent across seasons but it is possible to change your spin rate by altering your release or mechanics (that is to say there are examples of pitchers who have changed their spin rates going forward).
High spin pitchers tend to be more effective than low spin pitchers on a number of metrics (K, pop-up, etc), but a similar correlation is found when you look at dSpin per velocity. That is to say, since spin rate scales linearly with fastball velocity the further a pitcher is away from the average spin on a FB with the same velocity, the more effective the pitch is (think Koji Uhera FB, his spin rate wasn't silly high if I recall, but it was very high for a pitch with such pedestrian velocity).