Like a few have mentioned it very much depends on what you like, where you are going, and what your expectations are. We have been full timing for about 1.5 years as a family and use a variety of clubs, a few thoughts on ones we use.
Good Sam -- Tends to be 10% off for a night or two stay, anything past that is not discounted, but based on parks policy. Somewhere around 2000 participating parks if I recall. So not a huge savings, but the real benefit is they have pretty good online trip planning tools and you get a discount buying stuff at Camping World, which as much as I sometimes hate them, tends to be a consistent store for buying RV related items.
Passport America -- A discount club intended to help parks fill their spots on off peak times. Normally about ~50% discount on rates good for Sunday-Thursday (Depends on parks policy) and can normally use only a few days in a row at one park. Awesome deal for finding parks when you are traveling to a destination and don't want to Walmart it. Easily pays for its self in 1 or 2 uses and I think is a no brainer for most on the road. Does anyone know if Passport America is owned by Good Sam? I ask because they oddly share the same office building in Crestview, FL as Good Sams mail service.
Thousand Trails (TT) -- The beast we love and hate. A hot bed of confusing programs and information owned by a larger vacation/lifestyle company. Like others have mentioned some of their parks are absolutely run down, but on the other hand some are excellent. You will never find the truly unique interesting parks run by a family because it is a large corporation and the bottom line is king, but you can find some fun parks and a place to park your rig with full hook up.
TT parks are mostly down the east coast, across FL, TX, then from CA up to WA. If you need parks in the middle of the country it is a barren wasteland. We have been to some great parks in PA like their Hershey and Gettysburg Farm locations (great parks), but we have also spent time in their Indian Lakes/Batesville which is almost a 1000 sites and caters to Annual sites who like to party all weekend (not our cup of tea)
With that TT also has a host of options from Zone passes to elite memberships that allow access to all parks (~80). When we tried to find out info about it we thought it made sense to talk to a TT rep at one of the parks... not a great idea, they tend to not have much info on how TT works and are armed with an agenda for selling that benefits them. First time we went that route we got a lot of bad info. We did end up finding a blog that did an excellent job explaining how they work (search "RVLove TT" on Google and it will pop right up). Note we are not affiliated with them at all!
From there we learned that Elite type memberships from TT are lifetime and transferable, so there is a market for selling/buying old ones out there. But there are also 100's of different memberships and contracts out there so it can be uber confusing. So with that we go directed to
Buy and Sell Campground Memberships | Campground Membership Outlet who basically resells campground memberships, but best of all you can pick up a phone and talk to them and they will explain everything about almost any membership program (including TT) and better yet they facilitate buyng/selling of used memberships. We got an older Platinum Plus TT membership for around $2800 that gives us ~75 camp grounds we can stay at for up to 21 days, park to park and an annual fee of $545. If you buy the current similar package from TT MSRP is like ~$12000. For us it has worked great, when going to visit a location we love to explore, we don't spend a lot of time at the park itself unless it has something cool to offer. In 2016 we spent almost 260 days at TT, so it has easily paid for itself. Would take a VERY long time with the 12K version!
So those are just some of our thoughts, there are lots of options out there and to be honest it is hard to find good info on a lot of them, I would start with something like Passport America as it is a no brainer and talk to folks who use the different memberships and learn a bit before expanding.
As a side note this year we are spending a lot more time in state/federal camp grounds as we like the more rustic settings and don't mind W/E only. They also tend to be value priced plus a bunch participate in Passport America.
Have fun and good luck in your adventures.