Many people who built the beta version of the Printrbot Simple have experienced a sag in the Y axis, causing the print bed to be way off level. This upgrade takes care of that completely on my printer, and hopefully on your printer as well.
It fixes the problem in two ways. First, it provides a very secure, curved slot for the bearings, which helps keep them lined up. And second, it has a built-in 1.5 degree slope to counteract the sag.