I have spent some time calibrating my Printrbot Simple, and am having excellent results. It took some experimentation and a good set of calipers and about a dozen little test prints, but I think it's there. :-)
I found that though the X/Y/Z steps per mm were right on, I was overextruding by about 8%. This didn't show up when I measured the amount of filament being moved - in fact, with the stock settings, it appeared that I should be under-extruding. But the symptoms were consistent - my prints were about .3mm too big in all directions on the external dimensions, and holes and slots were printing too small. Dialing back my extrusion multiplier to .9250 did the trick - prints are smooth and fit together.
I also switched filaments to Zen Toolworks PLA in white, and I am very pleased. I found that in my cool basement I had to run it at 210C to get good layer adhesion, but once I figured out that, man, it's nice.
I found a simple way to hang a filament spool, which eliminates tangles on long prints. I had a wide hook designed for hanging things on garage walls that was the right size. I had binding until I removed the rubber coating, exposing the steel. Works great.