Actually, you will be renting a reservation that a member makes with the points that they own. You'll need to be comfortable with giving all the control over your reservation to the member since you will not be able to make requests or add things to your reservation, not even confirm it with Member Services. You can save money and get a very nice vacation, but you will have to put your trust in the hands of a member. First recommendation, find someone you feel comfortable with.
Hopefully, you aren't planning anything for anytime soon, even Oct 08 to early Jan 09 since a lot of the resorts are already booked. Members can make their reservations 11 months out, so many do.