Well worth it in my opinion but this is the kind of question that brings out the kind of people who will go negative because they dont have one and are antagonistic towards those who do. It's really quite strange.
Our experience; After 13 years our last TT still looked like new and the graphics were perfect because I kept it covered during the winter months.
The last winter I finally tossed the cover because it was nasty and we were selling the TT. During that time it aged considerably - especially the roof.
Because of the cover, we got thousands more for it than we would have if it looked like all the others I found on-line.
The only time I would not recommend one is if your in a very windy location. Otherwise it money well spent.