Hi Turb.
I've been in the Marriott scheme for a couple of years. You buy points now rather than a set week, which makes things a lot more flexible.
The upside is you can transfer your points to Interval International and get access to places worldwide ( going skiing in the US this Christmas on our points). Also you can book a place within 2 months of going and spend about a third of the normal points. We bought enough points for one week in peak season but this way we tend to get three weeks from our points per year. Plus there are a few other benefits like short stays for only $50US a night.
Down side is the annual fees can add up. $1000us plus per year.
Go along and enjoy the 3 free nights. The resort at Mai Khao and the beach club are both excellent. They sleep up to 6 so are excellent value with kids.