I own four weeks of Marriott Time Share that we purchased under the old weeks based system. Over all, very happy. I believe the new points based system is an improvement over weeks. Before you purchase, do your homework. There is a ton of information avaialble on the internet (not all of it is worth reading, but there is a lot available). Even with the new points based system you will still be paying maintenace fees every year. Make sure you can afford them.
Go to Timeshare user group for a lot of info.
I looked into it heavily back in 09 , but it didn't make much sense because you can go online and rent other people's weeks at the maintenance cost or below. We attended a presentation when we were in Hawaii in 06 and the upfront fees were ridiculous, these timeshares are now selling at a steep discount on Ebay, etc. Also with www.vrbo.com you can rent vacation rentals directly from owners.
With Marriott spinning off the Timeshare unit there is an element of instability with their properties.