This X Gantry and Carriage is Awesome! I've been using it for about a month now, and it has eliminated all my gantry sag, given me more printable space, and just looks freaking awesome!
I did run into a few snags though...
The teeth on the belt clamp on the X carriage did not line up with my stock belt, so I had to remix the carriage. You can find my remix here: "QU-BD_OneUp_XYSlimCarriage(Stock_Belt_Mod)" (https://www.thingiverse.com/thing:1656310)
I had a few other issues as well, you can read what they were and how I resolved them here: (http://www.thingiverse.com/make:234056)