Sprint is not here in Alaska. So you would be roaming. But they are partnered with Alaska Digital and ACS Wireless. So as others have said it will work. Voice and Data. I would say if you have a nation wide plan there would be not extra charge. But you might want to call Sprint to make sure your plan covers Alaska.
As for range. You will have signal in most larger cites and main road ways. But thats about it. Alaska is for the most part empty of people. (So they don't put alot of towers in the woods
) Only 600,000 people or so here. 300,000 of those are here the Anchorage area.
Your welcome to PIN (246E20F5) me if you like. I can give you more info if needed. Have a great trip. I love it here.