Usongshine MGN12 Linear Rail Guide with MGN12H Linear Bearing Sliding Block Match for CNC xyz DIY Engraving Machine (H-Type，250mm)

I didn't expect these to be any good given their price, but they are great. They have no discernable play, and roll very smoothly. They don't ship with any grease in them, so you should make sure to lubricate them.

I bought the 200mm and a 250mm C-Rail. The 200mm rail glidded smoothly. However, the 250mm rail had two sticking areas, right out of the packaging. I tried fixing it, but the rail continued to stick at the same two spots, like there were magnets. I wouldn't use it for X or Y applications, but maybe Z axis, or not at all.