I agree with some of the other reviewers that the road quality is horrible here. Any normal car will have a hard time negotiating the huge potholes and bumps. I would recommend hiring a guide even for the parts that you can supposedly drive yourself.
Others have already commented on tours, etc. It's definitely worth splurging to go into the "off-limits" part of the park. After enduring the hordes of tourists at the Grand Canyon, I was surprised at how quiet and uncrowded Monument Valley was when we toured it at around sunrise.