Have bought the adapter for my new iMac 27-inch (Late 2012), because he was a little too slow with the internal hard disk.
With the adapter, and the Samsung 840 256GB SSD, the system is now lightning fast. Reads of 360MB !!!!
My tip: investing 250 for the adapter + 840 SSD and you will be amazed how fast the iMac but can be compared to the internal hard drive.