Just say no!
They will make it sound like you'd be foolish to not buy, even if you don't use it you can rent it for a profit, or re-sell it, they are more full of crap than the port-a-potty at the chili fest.
That said, if you forget about your original investment, the maintenance fees can be significantly less than even hotel rates, and having a condo with a full kitchen can save you money on meals.
There are so many variables to consider, but unless you plan to go to the same place every time, exchange value is most important. If you buy at French Lick, Indiana, you will be lucky to exchange to Florida, forget about Hawaii unless you want to exchange your two bedroom for a studio or hotel room.
We're pretty happy with our three weeks every other year in Hawaii and our every year week that was originally in Mexico but now is also all over the US.
Somehow we're doing 46 days in Hawaii next year, with enough points left over for a couple more weeks if we want.