Best portable HD for Airport Base Station

I'm going to buy an AirPort Extreme Base Station as my next router and am looking for a good portable external hard drive to plug into it as my network drive. I've ruled out getting a Time Capsule because I've heard mixed reviews.

There's a number of 500GB portable drives listed on the Apple site and I was wondering if anyone has any experience with any of them with an AirPort Base Station. I prefer a portable drive because I don't want to have to plug yet another thing into an electrical outlet.

I was also wondering if these portable drives are made to be on all the time. I'm not sure if the drive will be on all the time since the AirPort will be on all the time or if it will just turn the drive on when it is accessing information. Thanks in advance for any advice!