1.) DVC is worth it if you plan on visiting WDW or your home resort every other year. You get the most bang for your "DVC buck" if you visit one of the 5 DVC resorts (HH,VB,OKW,BWV and now WLV). If your prefer to stay at Disney's moderate or deluxe resorts on points then IMHO it is not worth it. Having said that, most people really don't care about not staying in the other resorts when you see how much the DVC resorts have to offer.
2.) DVC is worth it if you purchase at the home resort where you'd like to spend the most time. If you plan on spending most of your time on-site at WDW don't purchase at HH or VB because the 11 month booking advantage is becoming more and more essential. BWV ressies in particular are becoming more and more difficult to snag at the 7 month window for certain periods of the year. If you are in love with OKW or the BWV, then don't purchase over at WLV. There are plenty of resales for OKW and BWV out there and you'll be happier if you purchase where you'd like to stay.
3.) The DVC is worth it if numbers 1 and 2 are true and you'd like to save lots of money over the next 41 years. We'll break even on our initial investment ($16,250) and the dues we've paid after about 5 years (we joined in 1999). Keep in mind that your break-even point may be further down the road if you finance your DVC purchase, about 25-50% longer according to our calculations(depending on how long you finance for). However, depending on how you finance your DVC purchase, your interest paid could become a decent tax write-off.
4.)Finally, DVC is worth it if you have some degree of flexibility with respect to your travel plans. It is not always possible for all of us to know 11 months out that we can take a vacation that far away time-wise. If you book before the seven month point then you should at least be able to stay at your home resort, but may not get the exact accomodation that you'd like.
We are really thrilled with our purchase to date and will probably add some points on at BWV (via resale) down the road. Right now our initial purchase of 250 points is working just fine, but now that our families are loving to journey down to WDW with us, I can see that eventually we'll want more.