I needed a rail system that is completely straight, and these fit the bill. The only critique I have is the bearings on the slides could be a little smoother.
I added this to a sliding table top that was sliding on dovetail slides, not smooth and wobbly. Now with this the table slides on two solid steel rods and ball bearings. Not totally smooth but exactly what I expected for the price. I'm happy. The shipping was super fast too.
Sturdy and better than square Linear Motion Guides
I use this pair suspended holding my Z-axis for my CNC Machine and have no sag or bowing when cutting. It is very smooth in my setup and has very little twist. I found these better than using the rectangle Linear Guides using MGN bearings.
Sylvia Bailey 
