This is a good link
http://www.exlar.com/products/linear...RolScrew12.pdf it's good some good pictures in it.
Looks like the standard nut has 0.01-0.03mm of backlash on it, but there is also a preloaded version. I am intrigued to know how it compares to something like this
http://www.kerkmotion.com/ which claims zero maintenance with zero backlash, but there are no real values quoted, but does look a lot cheaper.
Stephen