We stopped at the Costco at Iwilei last year while staying out in Ihilani. Being from Atlanta, it was just like any other Costco as far as crowds and parking were concerned. We found it useful for gassing up the rental car and picking up breakfast items for the family like cereal bars, juice, and snacks. Although not referenced in this thread, there's also one convenient to Kona.

There's one thing good about living in metro Atlanta, once you've experienced our traffic, there's no such thing as bad traffic anywhere else