I've been periodically in the same boat and thought about joining. You have some excellent reply posts covering the major issues, especially Charles's list.
My 2¢:
Pros: I would call it a good "emotional" investment if you will.
Cons:
1) If you are sure you will go enough in the future, it will probably be a wash. Good financially when economic times are good but poorer when times are bad. For example, DVCers can't benefit from Disney's discounts when they must discount, but will be protected versus increasing rack rates if paying cash. And it's hard to compare the deluxe hotels to DVC properties. Just apples and oranges when it comes to amenities and service.
People can avoid rack rates with
AAA or AP discounts (in good economic times). The AP room discount, if staying at a deluxe, will quickly pay for itself .) For example...
SSR studio
Holiday rate: rack = $425; AP Discount = $249
Value season:Rack= $295; AP Discount = $169
2) Disney Inflation: However, Disney continues to increase rates MUCH higher than the annual inflation rate, for rooms, tickets, food... and DVC annual dues.
DVC has annual dues increases about .50 - .75% higher annually than rack rates.
For example
1) Ticket prices:
2000: 1 day ticket was $46. now, in 2009, it's $84.
Are you earning 82% more now than 9 years ago?
Do you think you will earn 82% more 9 years from now? Disney tickets will be way higher than inflation....
And last year, despite
deflation in the U.S. economy from Jan - Sept 2009, Disney increased their prices about 4-5 %.
3) ...And Disney cuts and cuts, outsources and outsources, skimps and skimps. We just got back from a
Disney cruise 2 weeks ago. 1st example: The budget cuts really showed on the Wonder compared to when we sailed in Nov 2007. It took 2 days for us to get hot water in our cabin Cat 4 stateroom.

2nd example: While paying for top dollar for 5 MSSSHP hard-tickets, Disney gave a few cheap, poor quality candy to my kids. I mean,
generic Smarties?

They can't be serious. I'd be too embarrassed to give that stuff out to kids in my neighborhood at Halloween. And don't get me started about the
stroller rentals...
So basically, I've been seeing Disney raise prices more and more while returning less and less, especially over the last 3-4 years. After our last trip a few weeks ago, my feelings to Disney was just
less magical than the one before, which was less than the one before that. Maybe that is short term. But I've started to think about doing ski trips again rather than the next Disney trip. Tells me something.
Good luck with your decision. - Mike