I have three cat trees all of varying size and brand.
The smallest is a off-name brand 1' sisal tree, that I got at our nearby bargain outlet store. It's been used and abused by all the cats, and is still going strong almost a year later.
The medium tree, is about two and a half feet tall, and I think I bought it at Petco. It's covered in carpet, and after about 10 months of use, it's definitely on it's way to being thrown out as soon as I can find/afford a replacement for it. The carpet is pretty well shredded, and I find myself having to trim the tree with scissors once a week because of all the hanging carpet threads. The height is nice though...the girls like to bird watch from it.
The biggest tree, is a 6' Armarkat, that the girls got for Christmas. It's covered in sisal, and has held up wonderful so far. The tree is sturdy, and the only problem I have with it, is that it's white...with three dark colored cats, the hair really shows up!
I think if I ever buy anymore cat trees, they will all be sisal. It lasts much longer, looks great, and my girls seem to like it better.
You can see all three in this picture.