I have used this line of Wellness for one of mine who recently gained some extra 'flab.' I added some home cooked meats to it though, didn't feed it strait. It seemed to work pretty well for her. I made sure that she was a little more active though too--a few extra games of fetch and extra time outside chasing bubbles.
A food designed for weight control is find to continue feeding--normally they are just higher in protien levels. But exercise for lazy pooches is equally as important for helping them stay healthy.
As far as buying Wellness or any other brand from a chain store, I see no problem with it. I don't think Wellness in a bag sitting on a shelf at PetSmart would be any lesser or better "quality" than a bag of the same product obtained elsewhere. I'm doubtful there is any place that carries "freshly manufactured this morning" bags of food.
I have noticed that those brands are not usually carried in as high a quantity as the others in the store--not sure if that makes a difference or not.