It depends which formula. Diamond Naturals Active Cat is pretty good, and I use it in my custom mixture of dry foods. The Naturals Indoor Cat formula is VERY high in fiber, too high for everyday feeding of normal cats, IMO. I'm sure it works for some cats with bowel issues, though. The Naturals Kitten formula is good.
Regular Diamond Maintenance formula is nothing special. About the same quality as Purina Cat Chow (well, maybe a teensy bit better because it doesn't have any soy products or food coloring). Overpriced for the quality, IMO.
I got this information from the company, maybe it will help someone.
"Thank you for your inquiry.
The pH in all Diamond Naturals feline formulas is between 6.1 and 6.4. The Indoor formula for cats contains 0.10% Magnesium, as fed. The Active Formula for cats contains 0.12% Magnesium, as fed. The Kitten formula contains 0.09% Magnesium, as fed.