We're using these:
http://www.mcmaster.com/#6738k41/=vnxcue
http://www.pbclinear.com/Download/Da...ear-Guides.pdf [pdf]
They're kind of expensive at $60 each roller (for four), plus another $80 per rail but they will save us a ton of headache. I know from experience that linear bearings are hard, and bad bearings make it harder
This way, the elevator can tolerate some unbalanced loading, has zero slop, goes together easily (with versaframe) and slides with low friction, even under load.