Im really surprised Costco hasnt written their own app to do the same thing (in addition to serving up coupons, maps of store interiors, push "deals" inside the stores, food court ordering, etc.). In the absence of an official Costco app, this does exactly what it advertises. Its handy to be able to see which locations have gas stations and its very handy to be able to search for locations even with poor (or no) cell signal.
Id really like to see the app report store and gas station hours so I know if the gas station is still open. Id also like to have quick access to the direct numbers for departments inside the store. A "get directions via..." button would also be useful, as already mentioned by others.