Oh definitely worth it, mine spend most of their time on their cat trees.
I'd always recommend Kalven cat trees, because they're really sturdy (the posts are wood not cardboard like many) and the carpet is really good quality, lasts for years. Mine are over 10 yrs old now and one is just starting to look threadbare at the base (where they've scratched it).
www.kalven.co.ukThis is one of the upstairs ones:
This is the one thats a bit threadbare and the sisal around the posts needs replacing, but its the one they use more than any as its in the lounge.
They like to share too LOL!