Based on your trip history in your signature line, it looks like you've been traveling once a year and stay in deluxes... so...
Up-sides:
-You'll have deluxe accommodations essentially paid for during the next 36-50 years.
-You'll have the freedom to travel to other
DVC resorts outside of WDW (HHI, VB, any new ones that are developed)
-There are some member perks (though not guaranteed perks) - discounts on annual passes, discounts at some restaurants and stores, pool hopping, etc.
-You can use the points towards other non-Disney/non-DVC trips (flexibility is there, but argueably not a good use of points).
-You have the option to bank or rent your points during times when you can't use them.
-You can borrow from future year points to increase current year points and take a longer vacation (or stay in larger accommodations).
Down-sides:
-Large upfront expense
-Yearly maintenance fees that do change
-High point costs to stay at DVC resorts on Friday/Saturday nights
-Currently can't take advantage of some Disney programs (like the dining plan)
I'm sure folks will chime in with more ups/downs....