I just received my X-Sabre today. I have a Macbook Retina and the USB was plug and play just like the documentation said. My iTunes and Spotify sound good, and when I fired up Audirvana Plus and switched to my higher res music, it really came alive and sounded pretty fantastic; although I haven't tried my collection of amps/headphones yet. Right now it's just my HE-6's and Emotiva amp. :)
But, I did want to point one thing out though. I noticed that when a co-worker sent me a youtube video, there was no sound. I tried a couple of browsers and any sound that played in flash just didn't come through. While not super important to hi def audio, it certainly was annoying none the less. I went into the Audi MIDI Setup tool and noticed that it defaults to an output format of 384k. When I changed this to 192k, sound started playing through flash just fine.
I haven't really checked any other reason that would cause the sound not to play in flash at 384k, but this is a quick workaround for those times a Mac user wants to hear their youtube videos. ;)