My wife and I went two years ago. It's quite beautiful and very dusty, so be careful with your camera and lenses.
We went on a bus tour, so we had a lot of high sun. I believe a lot of the land is owned by Navajos, so be aware of any regulations or customs.
There are a lot of books and photoguides about the area, so do some research. Go to
this photoguide website to find some guides.