I did feed SD but I've since switched Sophie to Orijen. It's completely grain free. It does have a lot of protien, but that's not bad unless she happens to develop a kidney or pancreas problem which I hope she doesn't develop. Many people worry about Yorkies gaining weight on a higher protein food, but actually Sophie has stayed pretty much the same. Since starting to feed her that, she's gained normally and hoovers between 4.9 lbs and 4.8 lbs according to our scale. Also, her coat looks soooo much better (I feed the 6 fish formula) and her eye boogies are better too (as long as I keep the hair out of her eyes!).
So while the SD nature's best isn't probably a bad food, there are others out there that are better and the price is not much different.