I have a 2010 Mac Mini server running ML client 10.8.2. I replaced one of the 500gb drives with a 256gb SSD drive and using directions from the web, I created a 756gb fusion drive. 
It was easy and so far seems just as fast as the SSD alone. I just used the spare 500gb drive I had removed, and had put into an external enclosure. I then backed up my mini to the USB drive with Super Duper, then booted from that, created the fusion volume, and then used Super Duper again to back up the usb drive to the new fusion drive. Rebooted from the fusion drive and done!
It was easy and so far seems just as fast as the SSD alone. I just used the spare 500gb drive I had removed, and had put into an external enclosure. I then backed up my mini to the USB drive with Super Duper, then booted from that, created the fusion volume, and then used Super Duper again to back up the usb drive to the new fusion drive. Rebooted from the fusion drive and done!