I've havent seen two 500 GBs, as they are relatively new. But i just saw for the first time the hard drive replacement for the ODD the other day when one of the techs was working on it.
1. The computer technically has its warranty voided. Adding a 2nd hard drive I believe constitutes for "modifying functionality, etc." on the AppleCare terms. The owner probably put it in himself.
2. It was a pain in the butt because it had its logic board replaced, so we needed the disc drive put in so we could complete the repair (serial #). We had to call the customer to bring in his superdrive.
ODD > 2nd HD in my opinion.