Misty is right...it's good food if your cat will eat it. My cats do like most of the flavors, I offer it in rotation with a few other brands. I don't lean on it real heavily...partially because it doesn't seem to be as filling/satisfying as some of the other foods they eat...and partially because they do get bored quickly. My cats eat Wellness, Natural Balance, EVO, By Nature, and a few others in addition to the Merricks. I believe all of those are "all life stages" foods.
If I was trying for some weight gain, it would not be my top choice.
When they need to gain, the numbers (calories, protein, fat, etc) on the can don't matter if they won't eat it, though. Precious' favorite food is kind of low in calories & fat but it's the one she'll eat reliably so it works. I hope that makes sense.