We recently received one as a result of trading one of our resorts. It had a grid like mentioned earlier, so it was somewhat limited (ie. no high seasons from what I could tell). However, we were able to reserve a Marriott 2 BR unit in Orlando for next April and the cost was $349. A 1 BR or studio costs less. A regular exchange fee is $149 as I recall, so we basically paid an extra $200 and received what we think is a nice unit.
Some of the areas available were international as well.
So depending how flexible you are and where you want to go, it can be a good deal. But, it isn't a free week other than you don't have to relinquish another one of your own weeks.
Good luck.