PetCo's site is jam-packed with too many images, links and special offers. If the object of a design is to make the user comfortable, this site fails miserably. That said, after a few minutes, I found it relatively easy to locate everything I might want.
Items are categorized by type of pet, so you can go right to where the thing for which you are looking will probably be, but it's easy to get side-tracked by all of the "noise". Visually, the homepage is very cramped and it just gave me a cramped feeling looking at it.
One thing I liked is the fact that their "Store Locator" is right up front where it is very easy to find; I don't usually see on other websites.
