I agree the best thing about being a member is access to the reviews. They seem much more comprehensive than those on other sites, and I feel confident that they are submitted by real people who visited the resort, and for the most part are unbiased.
Trade test results are also posted in the sightings board which can give you some insight into the trade power of your resort(s).
The historical sales area may also be helpful if you're looking to buy or sell a timeshare -- members can post details of timeshares they've purchased here to give you a general idea of what the value may be.
You also get a free classified ad with membership. If you were planning to place an ad for a sale or rental, you may as well become a member.
Welcome to TUG regardless