Head to the Dis boards for way more information than you'll get here.
My impression is that it rarely makes financial sense - and especially not if bought direct from Disney - but some people do find value in being "forced" to vacation regularly by having precommitted the funds. Personally, even though I love Disney and am currently going about once a year, I'd rather not tie up the money. Rent DVC points from a member when you want that kind of room, stay at non-DVC resorts when you don't need a villa, and heck, vacation elsewhere when you feel like it without having to worry about using/banking/renting out your points.