If you puchase your full allocation of General Admission tickets and camping tickets, the savings actually are more than the cost of the membership. Plus you then get your free programme, results list, etc etc.
If you go to more than one event (i.e. the Classic, Moto 24) then you save even more!
But no, it doesn't guarantee you the camping that you want (as I have discovered for the last two years!)
One other tip, join around May time and buy your GA tickets then! Then, order next years tickets before the renewal time and you'll get two years of discounts for one membership year. Then don't renew until the following year!