We went on Valley Tours during a short (2 nights) stay in Nepal. They were both great experiences, and we were lucky enough to see a lovely historical display.
The guides (Ashok) was really enthusiastic and very knowledgeable, and the other members of White Stone that we met during the course of our stay were all also very helpful. The whole company has an air of laid back efficiency that seems typically Nepalese (to someone from the uptight, inefficient UK at least!).
If you are visiting Nepal, I would definitely recommend a visit to White Stone Holiday, Lovely people and fantastic things to see. I you do visit, then I'm sure any of the many tours/excursions that they offer will enhance your trip (and in fact your life).