I suggest the book Timeshare Vacations For Dummies by Lisa Ann Schreier. She used to sell timeshares, and her book explains many details that new purchasers don't understand (and perhaps don't know to ask). For example, she discusses the difference between owning points and owning weeks, she discusses how to trade locations, etc.
Most importantly -- unlike the timeshare salesperson -- she isn't trying to sell you anything. She's unbiased. I think people who are unhappy with their purchase are usually the ones who didn't look into the timeshare world closely enough, and especially those who assumed that the salesperson was a friend looking to help them get a nice property, rather than a salesperson looking to make a sale. The timeshare people do work very hard to look this way, and their presentations are seamless assaults on your emotions and your pocketbooks.
In my opinion, there are a couple unbreakable rules when considering a timeshare purchase:
1. Tour all the big-name properties, but REFUSE to buy on the day you tour. You owe it to yourself to be well informed before you jump in, and you can get some nice deals on short vacations.
2. Never, ever buy "new". The resale market is so much cheaper!
3. Never, ever finance a timeshare purchase. It's a luxury item. If you can't buy it outright, you can't afford it.
4. If you want to trade for other locations, choose a top-notch property that other people will want to rent; if you get a older, hole-in-the-wall place, you have no trading power. If I remember the book's advice, Orlando and Vegas are good bets.
5. Consider how you're going to want to vacation in the future; consider what you're likely to do when your children are out of the house, when you have grandchildren, and so forth. For example, once we really thought about it, we realized that the first timeshare that caught our eye wasn't really for us -- it was wonderful, but we really wanted something on the ocean.
6. I'd only purchase a timeshare within driving distance of my home. It makes it more likely that you'll be able to use your "home property" every year.
My personal opinion on the best product out there: I like Marriott's two-bedroom lock-out properties. They give you the flexibility to either use your two-bedroom property as is, or you can "split" your two-bedroom property into two one-bedroom one-week stays (for a $99 fee). On the other hand, my parents, who are retired, are completely sold on their Hilton timeshare because it's on points, and they have the flexibility to travel only mid-week when their points stretch farther.