I've had Cheeseburger in Paradise in Waikiki, and although it was very good, I'd say that Kua 'Aina was better. I don't mean to hijack the thread, but as long as CIP was mentioned, and I know a lot of people on the board are travellers, I thought I'd mention Kua 'Aina.
They also make a mean Mahi Mahi sandwich. If you are ever on Oahu definitely try Kua 'Aina on either the North Shore or near Ward Center, not far from Ala Moana Center Shopping Center.