There is a grading system which some folks use. It is a guide, a good place to start. I posted about it here:

http://petoftheday.com/talk/showthread.php?t=149509

Two varieties of Science Diet are rated (scroll down). They both came up with an "F" score. So, this means that under this system, there are better foods out there.

I used Science Diet for my cats for years. It was recommended by my vet, and I thought everything was fine. Then I found Pet Talk, and ended up doing some research on my own. I now feed Chicken Soup for the Dog / Cat Lover's Soul, and the pets are doing much better.