Started with 300 at Saratoga in 2005 and purchased another 100 at Animal Kingdom Lodge in 2008. Based on when we travel and the fact that we like to go with friends from time to time, this seems to be the perfect amount. Depending on whether or not we get our waitlists this holiday season or not, we'll either have 38 points to bank, or 16.
Haven't regretted the purchase one bit, and consider the additional discount on annual passes as a 10% savings on the annual dues.