Precision potential???

I see a lot of home-made CNC projects around the net using
stamped sheet-metal drawer guides for ways and I wonder, how precise
can this be( I am talking about absulute reapeatable precision, like
0,02m +/- 0,01mm variation(meaning it has a max precision potential
of 0,03mm))

What is the level of precision one can obtain using these things for
ways?????

San