I've been, but not to photograph (it was around 7 years or so, before I got into photography). My memory isn't the clearest, but I recall being very impressed by the karst formations down the river, I think they would make excellent landscape subjects. The "Yangshuo Moon Hill" was also pretty interesting.
On the other hand, it was quite touristy then, and the boats don't tend to slow down or stop as you're going down river (which is the easiest way to see the karst formations) so you may have your work cut out trying to get good composition & good light etc etc (can't remember what times the boats ran).
Anyway, hope that helps. If I were doing it I'd take a Rolleiflex and a bag of TX, and try to get out at sunrise/sunset. And see the cormorant fishermen too, it's quite an art.