We are DVC members and have been since 2009. By December this year we will have broken even on our purchase and then it will only cost us our annual dues each year. I have kept a record of the costs for our stays including checking to see if I could have got a discount and applying the lowest cash cost to it.
Whether it make sense for you depends a lot on where you stay now or where you actually want to stay.
We have BLT as our home resort and really only use our points there. We have stayed at BCV & BWV on our points too.
We love and stay at OKW but I book either the UK free dining & discount deal or a room only discount. I don't use our points for these stays as it is not cost effective at the moment (plus we don't have enough points and these deals don't make it worth buying more yet).
We also only stay in one bedroom units - we won't stay in a normal hotel or a studio for more than one night. This makes DVC make more sense too - you also get a full kitchen and laundry!
If you are happy to stay at OKW/SSR - then carry on with the cash deals while they last.
If you want to stay at one of the other DVC resorts then consider DVC but things to remember you have to buy food and tickets on top of the cost of DVC. There are currently AP discounts for DVC members and water park discounts but none of the other main park tickets. The other thing is DVC members do not get daily housekeeping - it is every 4 days and alternates between a full clean and trash & towel service. Not an issue for us but can be a deal breaker for others.
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.
Just one word on the RCI deal, when you check with RCI they have lost of resorts in France. When you check about booking through Disney, mysteriously there are none! I guess it is to protect their hotels at Paris.
You should check out some of the resale agents, they are substantialy cheaper than buying through Disney, only change is you cannot book the fancy Disney breaks with a re-sold package, not much of a loss as the cost is about 5 years worth of an average holding.
Best value we found was using them to book a Cruise with DCL, not too expensive points wise for what you got and at the time if you did not have enough points you could use a cash top up.