What always has amazed me in the history of the Givon battle is how God used the humility of the Givonites in bowing before the Israelite God to call together the strong confederation of southern city kings to do battle. Because of the Givonites, Israel did not have to fight prolonged wars with the southern kings, they were defeated in a day and all Israel had to do was mop up after. In a short time they had taken Jericho, Ai, Givon and the southern district with the exception of Jerusalem. What a confidence builder in God's promise to go before.
This will probably surprise you, but from what I understand (and it's been a while since I read this) there was an element of deception in what the Giv`onim told Joshua (they presented themselves as wanderers from afar rather than local residents or something like that; I need to check). For this reason the covenent Joshua made with them was not Halakhically binding, but he kept it in order to sanctify G-d's Name by doing so. Because of the element of deception there are only two things that will not be completely restored in the World to Come: the Nachash (the Serpent) and the Giv`oni (Gibeonite).
But the Gibeonites were "hewers of wood and drawers of water" for G-d's Holy People, and that was certainly better than being a pagan king.