Unless you have a spot that you know you want to return to many times, I would say no. Personally, I prefer to try new places each time. Even if you get a time share that allows you to pick from a pool of destinations, you can still typically come out better finding a deal on priceline or travelzoo.
Like Gabe said, you will be stuck going to the same spot everytime. Also, many timeshare companies are scammers so be careful.