UHMW slides

I have a question. I am building a CNC (hobby) and need it as
accurate as
possible. I am using aluminum extrusion from www.8020.net. They
have these
linear bearings which uses UHMW instead of ball bearings for slides.
These
slides uses slippery plastic like pads, instead of ball bearings, to
glide
along the aluminum extrusion. Does anyone have any experience using
these
types of slides? They said a shim can be purchased to provide a more
exact
fit with the slide and the extrusion beam.

I don't want to waste my money if these slides are not worth it. I
plan to
do PCB (circuit board) work with my cnc so it needs very good
accuracy.

Any advice or suggestions are appreciated.

Thanks,