My wife and I are Thousand Trails members and use it fairly extensively. We're lucky in that our membership was handed down thru the family and it's a national membership vs. one of the zones.
Here's an overview of Thousand Trails
that I wrote from getting questions about it.
And, here's a thing I wrote to help decide if TT is right for you
I've also reviewed 2 of the parks that we've stayed at.
Basically- you have to decide if the parks are places that you would like to stay and are in locations that are convenient to you and you'd want to go. $500/year is a lot to shell out if you don't use it, but can make nightly stays cheap if you use it to it's fullest extent. It definitely helps us offset our cost of camping by pre-paying for our camping each year.