To be blunt: its utter crap. Lots of by-products and fillers. By products are hard to digest and leave your cat with extra smelly and wet poop. Purina also lacks real good solid protein as main ingredients so your cat tends to eat more and get heavier (think of Purina as McDonald's - you eat lots more because you don't feel full since all you're easting is junk. Then an hour later you're hungry again)

With a premium food, they'll eat less. They will poop less and their poop will be more solid and a million times less stinky.

Also, one thing my rescue noticed: on Purina Cat Chow Indoor Formula, cats had a TON more hairballs.