I've had the X-Sabre for about a week now and I'm seriously impressed. It produces fantastic detail and separation in the sound, but manages to do so with a very analogue and smooth presentation. I chose the X-Sabre because I'd heard it was a musical DAC and it hasn't disappointed.
Build Quality
Ridiculously good! It's made from a solid block of aluminium, has a sleek look with simple white LED indicators on the front panel and a smooth finish to the exterior that looks and feels very high quality. It sits on 3 feet instead of the usual 4. This means it can tilt at the back corners when you're connecting cables, but it's a minor issue.
Connectivity
No Toslink might be an issue for some as mentioned in Project86's review, but I bought the X-Sabre for computer audio. It also has Coaxial and AES if required.
I first used the X-Sabre with a friend's Mac and it was literally seamless plug & play with no driver installation and no hiccups. Connectivity with the PC was almost as simple and just required a driver installation which is provided on a 32Mb USB stick in the X-Sabre box. Once connected and recognised, the X-Sabre works with both WASAPI and ASIO, but the ASIO driver is apparently the recommended communication method.
My last DAC would often cause issues with MediaMonkey when changing from lo-res to hi-res tracks. After over a week I've had no such issues with the X-Sabre and it's been a week with plenty of listening.
Outputs
The X-Sabre has a pair of RCAs outputting 2.2V and a pair of 3-pin XLRs putting out 6.8V. The RCA outs are fine, but the high voltage of the XLRs mean that there's no room for adjustment on the volume control of my amps - it's loud or off. I'm currently working on ways to trim the voltage using an L-pad design (resistors in parallel and series to bleed off some voltage), but it's a shame to have to fiddle with the system like this when Matrix probably could have built in a similar system to tame the super-hot outputs.
Sound Quality
The sound from the X-Sabre is incredibly detailed, but also smooth. The background is beautifully silent which allows the instruments to "pop" out of their well-defined space in the soundstage. Because the presentation is smooth and almost a little analogue sounding, the brilliant detail and high levels of definition between instruments don't become grating or fatiguing, but just exciting and engaging.
I wasn't sure what to expect with the X-Sabre having heard that it was a "musical" rather than analytical DAC, but it's only just a hair towards musical so it's not at all bloated or smoothed over like some "musical" presentations can sound. For me, the X-Sabre is the perfect balance between detail and enjoyment.
My detailed review and images are here: http://passionforsound.wordpress.com/2013/12/05/matrix-x-sabre-dac/