As others have said, it makes much more sense in the Southern climate where the sun is more intense and temps are higher. We just finished building a new home in FL, and many of the new homes put radiant barrier on the underside of the roof sheathing. We decided to go a different path. We sealed our attic space (no ridge vents, soffit is sealed, no other venting) and installed Icynene. This is a special type of spray-on foam applied to the underside of the roof and between any wall studding before drywalling. You do NOT insulate the floor of the attic when using Icynene.
On the hottest days of the year, the attic space will not get more than 10 degrees warmer than the living area. Though I haven't been in the house during the summer, yet, Southwest Florida can still get days in the mid-80's during the winter. I've been in the attic in the middle of the afternoon when the outside temp was 86 and the sun is shining, and could barely notice a temperature difference. You also don't have to insulate the duct work for HVAC since the attic air doesn't heat the ducts. As a bonus, besides energy savings, the house is much quieter because of the Icynene insulation. Plus, being in hurricane country, the sealed attic space won't allow wind penetration into the attic when the wind blows. My builder is so impressed, he's doing it in the new home he's building for himself.
For those interested, you can read about Icynene
here.
Kevin