Recently, one of my cats turned up his nose at the last of a 15-lb bag (he'd lunge for my roommate's cat's iams, though). I compared the ingredients list and analysis with the previous bag that I had folded up and put in the bottom of the closet where I keep the food, until I could remember to throw it away. Zero difference between the two. My other cat was still eating it, though.
My boy lost so much weight I tried the Solid Gold Indigo Moon (gotta love premium cat food marketing) and Before Grain's salmon flavor , and he'll eat both of those and has plumped back up again, so I think I'm going to stick with them.