We bought into one of them about 10 years ago. I think it was a 7yr contract, I know it's been paid off for a little over 3 years.
We originally got it because there were quite a few local NW resorts in the system and we could do weekends pretty easily and after our points were used we could rent condos for $25 a weekend. It wasn't too bad of a deal. Then the company got sold/merged with a different timeshare group.
On the original plan we had enough points for about 7 nights of a 2BR each year. After the changeover, we ended up with an average of about 17 nights, but we lost a lot of our local resorts.
We skipped using it one year and saved the points, the next year we took family to Vegas and got 4 2BR condos for 6 nights each a block from the strip and still had points left over. It worked out pretty well.
We still have to pay about $400 a year for "maintenance" but that's pretty minimal.
I doubt that it's been super cost effective for us so far, but it should start being better the longer it's been paid off. Where it's been good for us has been that it sort of forces me to take a vacation. Before we had the timeshare, I hadn't taken a full week vacation in over a decade. Now I feel like I have to in order to get my money's worth.