Well, if it's not the best doctor, it can give you some indication of whether you are drinking enough water. Just remember, when looking at the chart, if you are taking certain types of vitamins, the color can be a deep yellow for a few hours after taking it.
Water has always been something I need to force myself to think about. Drinking it is not natural to me so I'm not as good about it as I should be. But, I have noticed that when I drink plenty of it, I'm on the very low end of the chart.
You can also find a great chart on the US Army website here: http://www.al.ngb.army.mil/PublishingImages/Safety/258_Urine%20Color%20Test_Poster.pdf