Kirkland (a Diamond dog food)is an OK food for dogs...it's not the best, quite in the middle, but MUCH better than Purina, Iams, Science Diet, and the like...as far as food shopping at CostCo, this is the best food available at that store. Diamond is the choice of MANY quality breeders (I wouldn't feed it, because they don't use preservatives, and they did have a mold "issue" a while back...it did kill some dogs.)
I know many people on this site rely on the site dogfoodanalysis.com...while it's a great reference tool, I have been unable to find who the authors of this site are. I have a hard time putting stock in anonymous sources...they may not be hiding anything, but I, personally, have not been able to figure out who the admins. are. Does anyone out there know who publishes this site? I'd really appreciate the information...if I knew their credentials, I'd probably trust their opinion a little more.
I'd much rather see someone feed Kirkland than 95% of the other foods available. There are a few things in the ingredient list that I don't like, but overall, I would say, go ahead and feed, with caution...watch your dog for signs of allergic reaction, skin and coat condition, etc.
The best source of nutrition information is NOT yahoo answers, it is a combination of your breeder, your veterinarian, an animal nutritionist, and of course, your gut feeling.