Are “green” hard drives really all that green?

More and more drives are being sold as “green” drives, usually running at 5400 RPM rather than 7200 RPM.  Are they really energy savers, and what might you lose by choosing the “green” drive?

Ars Technica opens the question here, noting that the projected use of the drive weighs heavily on the decision to employ an green or non-green drive.