This is a loaded question and depends completely on your situation. How often would you go to Disney? What kind of accommodations do you prefer? How many nights a year?
If you go every year, the math clearly works in DVC's favor. For example, if you were to stay at Disney's Animation Resort (value resort) every year for a week, it would probably cost you about $200 per night (depending on the season), which would come out to $1750 per trip. In 20 years, it would cost $28,000, (assuming prices don't go up, but we all know it will so the overall cost would be much more).
If you go DVC, say you purchase a 150 point contract at Bay Lake Tower, which is a very deluxe resort. dvcresalemarket's last average price report from May lists BLT at $178pp (which is kinda high). That contract would cost you $26,700. Additionally, 20 years worth of maintenance fee would cost $20,700 (MFs will go up every year by about 2-3%). This would equate to about $2370 per trip at around $339 per night. But this would be staying at a deluxe resort instead of a value resort. Deluxe resorts tend to cost anywhere from $500-1200 per night, for a studio!!
So bottom line is, if you're ok with staying value and off property resorts, then DVC is not for you. If you like disney deluxe accommodations, then it's no brainer in my opinion.