Concerning the question about how long it takes to be "worth it" or "make sense" or "have value" or however you think of it... every family's story is different. Our purpose was not necessarily to save money. We find the value of
DVC in the way it has changed our vacations: for a family of 6, 2BR suites with family rooms and kitchens are so much more relaxing than connecting hotel rooms. It also has encouraged us to go more often or for longer trips, without the extra worry of the cost of lodging.
Some people don't want to be locked into
Disney vacations every year. But we live and work overseas, and when we return for a trip to the U.S., Disney is almost always something we want to do as part of a larger vacation (usually tacked on before a cruise). So DVC really made sense for us, and it had very little to do with saving money. We figured out how much we could afford and went with it.
This is the first year since we purchased in 2010 that we won't be making a Disney Parks trip, so we rented out some extra points through Dave's rental site. It was very easy, and took off any pressure to use up all our points. At least it pays for the dues.
Note that if you intend to use points at Grand Californian, you need to own there. ESPECIALLY if you are looking at going around Christmas. Its a very small resort, its the only place to use points at
Disneyland or the West Coast, and its very difficult to book at the seven month window.
There are other availability issues you should research to make sure its a good deal for you, but that one sticks out from what you said you intend to do.
I'd say this is probably true in general (not always, but more often than not). We are mainly WDW people, but wanted the option of visiting Disneyland every once in a while. We purchased Grand Californian points so we would be sure to have 11-month priority when we did need it.
As for WDW, there are so many options, we felt that it didn't matter to us which one we stayed at. As it has worked out over the years, we have managed 7-month reservations at BLT, BCV, AKV, and SSR. We will keep trying for BWV, and will now start targeting VGF. But if we end up at SSR or AKV most trips, it doesn't worry us. We have not found a resort we don't like.
BLT is probably our favorite right now, but we always intended to stay at as many different resorts as possible. So the home resort option is not nearly as important to us in WDW as it is in Disneyland.