I have been using these Drives for about two years now and they have performed better than advertised. They go together quick and the parts fit just like they should. I plan to use these when I build my next machine because they simply work well.
When I helped some student Make a CNC Router of course we used CNCRouterparts Rack & Pinion drives. Here is a picture of the completed gantry. http://www.makermasters.com/wp-content/uploads/2012/05/IMG_5- 057.jpg.
If I had to come up with one thing to improve it would be the turnbuckles. Although they work well they feel a little light duty. I have had no problems with them but honest opinions help make products better.
Hope this helps.