The original HDD crashed.
Very smooth. The how-to was right on the money. I chose to fully remove the logic board and blow out the internals with a can of air while I had it open. Was also able to finally rescue an SD card that had been lost to the Superdrive for a couple of years.
Take your time. I used a small tackle box with dividers to keep the screws sorted. I also used some small plastic blocks, about 5" tall, to put between the display and the case while unplugging and plugging in all the connections. I also covered both sides of the glass and display with a clean towel while they were off.