I have to say, if you travel on the spur of the moment, you are going to have difficulty with
DVC unless you are very flexible. DVC is really not designed for spur of the moment.
DVC owners have an 11-month booking window at their home resort, and a 7-month booking window at all other resorts. To give you an idea, we are going in December (booked in April) and are now planning a May '06 trip. Some times of the year are just plain impossible -- the first two weeks in December just about anywhere, and the Food & Wine period at BWV/BCV.
On the other hand...if you call MS and say, "I want to come next week, do you have ANYthing available," you might get lucky. You might be looking for a BWV standard-view studio, and get an SSR 1 bedroom...but you'd get in.
Again, DVC is designed primarily for people who visit at least once every other year, typically stay in moderate or better resorts onsite, and have the luxury of being able to plan ahead. Doesn't mean you won't be able to get anything, but it's not designed for that.