We have been owners since 1998. When we book ahead no more than 6 months out we have never had any problems finding what we want.
The resorts have been the most pleasant experiences in travel we have ever had. The problem most have is not booking ahead. Many think they can just pick up the phone and book a beach resort during the summer or a ski resort in the winter anytime they want.
It just doesn't work that way. "Proper planning prevents poor performance" I'm sure there are bad experiences by some, but I think many buy into a timeshare for the wrong reasons and have not thought it through.