We first bought some Cat Climbing Towers for the roof and balcony in Spain as we wanted something that was tough both in terms of surviving the climate (a lot of sun) and a lot of cats. We were not disappointed at all and even bought some for the cat rescue Sands works with and it is fair to say they get a lot of use!
The best thing as far as I am concerned with the trees is the strong hemp rope that is used to allow the cats to scratch. Ours has hardly frayed at all after a LOT of use.
They are all well designed and the cats love them: they can access them easily and move around on them.
When we moved to the UK we left the original ones in Spain and bought new ones. We designed a couple ourselves again, and also bought some of the wall shelves and internal trees. As before they have excelled and they are really worth looking at.