I've been a member since 2010 and have just added on with more points at a second "home" resort. Your "home" resort can be booked 11 months out and all others at 7 months so it's best to buy at your favourite resort, particularly if that resort can be more difficult to get into at 7 months put (BLT, BWV, BCV, VGF). Booking from 11 months works well for us UK travellers I think as we tend to plan/ book holidays further out than our US counterparts.
The scheme itself is very flexible and the points you receive year on year can be banked (saved to the next year) or borrowed from the next year giving you access at any one time to 3 years worth of points. We've borrowed in the past to have a blow-out 3 week trip over Xmas/New Year.
You can change resorts, room sizes, room views and dates each year so in 2015 you could have 2 weeks in a 1 bed lake view at BLT in October and the following year you could take the whole family in a 2 bed at OKW for 10 nights in March and then maybe a week just you and your partner in a studio at VWL in September. You are only limited by how many points you own (a big "only" I know).
You do get benefits as members in the form of discounted dining, tickets and special events from time to time, but ultimately you don't buy in for those and they can change/ disappear at any time.
I've never regretted my decision and in fact, it was one of the best things I've done. On my trips growing up we always stayed offsite, but despite discovering the benefits of onsite after staying in the values/ mods, we couldn't contemplate the spend for 2 weeks at CR for example. We then stayed at SSR twice on free dining and loved the extra space of the units and the feel of the resort. We did the tour again (having previously done it in my teens at BW) and once BLT was announced, I knew it was time to take the plunge. The buy-in is expensive, but compared to what we'd have to spend to stay on cash, it's great value. I financed my purchase but paid it off within 3 years and now it's just the annual dues I have to find.
I haven't used my points for an RCI trade or one of the collections, but I like that they could get me a trip anywhere in the world (dependant on availability).
Definitely research all the resorts to find which one is best for you and think about resale (the Timeshare Store is a good resale agent) particularly if you're looking for one of the sold-out resorts.
Best of luck
