The drive, RE4-GP, has what WD claims is "next generation" green technology, saving up to 40 per cent more power than normal drives. The cache is claimed to be good for a 25 per cent increase in performance as well, so this appears to be a win/win before you even consider the massive size.
The drives, available now, are going to retail for $329, or just under 17 cents a GB. For reference, this is slightly below the $10/MB I paid for my first HD. You can read more about it here.