We have had a lot of experience with TT, some good, some not so much.
My parents bought their first TT membership over 20 years ago. For them, it worked very well, as they lived full-time in their Class A for several years, and developed a system of travel from park to park with a group of friends every winter and work-camping in the summer. They upgraded from time to time over the years, and eventually passed the membership to me and my husband. It is an Elite membership which includes all zones, no time out between campgrounds, up to 21-day stays, and a four-month advance booking window. We have been using it for about 5 years now.
I believe hermanmullins is spot-on when he says that if you are not a full-timer, it might not be the way to go. It's pricey if you're just going to use it once in a while. Wildebill308 is only partly correct about the time out between facilities; with the Elite level membership that restriction does not apply, and you can stay up to 21 days. So if you would use it a lot, it also pays to have the higher-end level membership. Several posters have mentioned that TT campgrounds are not very well-maintained and the spaces are narrow and cramped. This can be true, and for a larger rig (we are 40') it can be a challenge to negotiate some of the campground roads. But have also encountered many that, while dated, are clean, navigable, and well-managed. They're not five-star resorts, and the sites aren't always level, either. Despite these drawbacks, we have developed some standard TT "favorites" that we use when traveling from point to point or just to kick back at for a week or so. For example, LaConner, Washington was a real treat, and we can always count on a clean, reliable spot at Three Flags in Florida. TT does include some higher-end resorts, though; their Encore campgrounds, but not all Encore campgrounds are in TT. Our membership gives us a guaranteed 30% off Encore parks not in the TT system (again, an advantage of the higher-end membership). We use the Encore option from time to time, especially when we are heading to a specific destination to stay for only a few days.
Most TT campgrounds claim to be near major attractions, but "near" can be over an hour's drive. Therefore, I would only recommend TT to someone who has plenty of time and is "nimble," meaning that they either are small enough to unhook and travel to sightsee on a daily basis, or have a toad. If you are the type of traveler that likes to pick a place, set up camp, and use it as a base from which to explore an area for a couple of weeks, (that's us) then TT might be a good option. But if you prefer to go to a specific attraction, stay a day or two to see it and then move on, it probably won't work well for you.
TT campgrounds also tend to be very family-oriented, so if you aren't into sharing the campground with children, you may not have a good camping experience. However, if you are a young family that wants to go to the same place for vacation every year, over time you'd likely develop friendships with other TT families that will be there at the same time as you are, which could be a real plus.
The other thing I would say is that in all honesty, the higher the membership level, the better your experience will be. There are restrictions on the entry-level memberships, especially the shorter booking window. With the shorter booking window, you might not even be able to get into the TT campgrounds you want at peak season (I have friends with an entry-level membership that have told me they have never been able to get into LaConner, for example). Some of the posters above have mentioned that all the best sites are gone or they can only park up front. Or, as Five stated, they seem to be filled with vacant trailers. I have also found this to be increasingly true. I suspect it is the campgrounds' effort to stay afloat that causes them to sell seasonal sites which then, of course, leads to the place looking shabby. We try to avoid these places, but we understand that those folks just prefer to camp differently than we do. I have never felt unsafe in a TT campground, but I have felt blessed. And, as Wildebill308 points out, this situation is typical of membership campground packages.
Most of the time, sites at TT campgrounds are not assigned, you just drive around until you find one you like. We find we have better luck at getting a nicer spot if we arrive mid-week. Again, if you don't have the freedom to do this, you're going to have a less satisfying experience. Arriving at a popular TT campground on a Friday afternoon in season is not for the faint of heart.
Not all TT memberships are the same. Every membership is a negotiated contract. If someone was really interested in getting one, I would suggest trying to connect with a private individual wanting to sell a higher-end one. I have heard of these things going at reasonable prices when they transfer between private individuals. I would also suggest that if you find a seller, look at the specific terms of the contract. Not all Elite contracts include the 21-day stay, for example. In the end, it's kind of like owning a vacation timeshare. If you don't really use it, it just isn't worth it. One other caveat: if you aren't careful, you can let it drive your travel planning. You might begin to reason as follows: why spend $45.00 a night, when we can camp free (except that free is 30 or even 50 miles from where you really want to be)? Sometimes, we go ahead and spend the $45.00 (or use Passport America, as Wildebill308 suggests) because we only want to be in that specific area for a couple of nights. Free isn't always better. Where it pays off is when we want to be, say, near the Smokies for a couple of weeks to escape the heat. Or go explore the Oregon coastline for a month. Generally, we find that it does pay for itself on an annual basis, but we travel frequently.
Bottom line? If you aren't on the road a lot, if you are a "destination" RVer, or if kids drive you crazy, it's probably best to pass. But if RVing is a major part of your life and home is truly where you park it, then it's worth considering. But try to go higher-end if you do. Happy Trails!