I'm using a WD10EACS as a backup drive. The drive is used along with a Vantec Dock and a nice small freeware backup app about once a week.
Observations:
Quiet. Can just hear the drive spin up, but no more noise after.
Cool. Under severe load the drive doesn't break 45 degrees. This is on the dock people, so zero airflow. Under idle, the drive doesn't go above 30 degrees. It spins down to conserve power quite frequently (once in half hour), but I also noticed this in my 640AAKS, so it's no major problem.
Fast. I don't notice a performance drop from the 750GB AAKS source drive, backups to and from take the same time. It is not benched, but as long as it doesn't feel slow I'm fine (and it doesn't, at least not through eSata).
Prices of all components in the market have hit the roof since the dollar appreciated. Wait and watch is advised, but not too long as the dollar isn't headed south anytime soon. Also some dealers will want to book profits before Diwali as propensity to spend increases during this season. Hang on till a little bit after Diwali unless you have no option.