I have heard many complaints about how the Z axis of the Prusa I2 and others, hangs from the motor shaft coupler and can often slip due to all that weight pulling it down. At least one person has had their X axis fall off completely.
This shaft coupler is a modified version of one presented by Rolller with the difference being that it has two high strength neodymium magnets locked inside of it.
To use it simply allow the motor shaft to come into contact with one side of the magnets, and the lead screw to come into contact with the other side of the magnets. The magnets alone should be able to support the weight of the Z-axis and print heads even before you tighten the screws to lock the shafts to the coupler.
The ends of the metal shafts should be as flat and perpendicular as possible to ensure that they make strong contact with the magnets.
Here are some Things that I've found to be useful for RepRap Prusas.
Please add more in below!
Belt Tensioner http://www.thingiverse.com/thing:10082
Quick-Fit X Carriage http://www.thingiverse.com/thing:19590
Prusa FanDuct http://www.thingiverse.com/thing:51611
Light Ring Mount http://www.thingiverse.com/thing:16164
NEMA 17 Fan Mount http://www.thingiverse.com/thing:29572
RAMPS Mount http://www.thingiverse.com/thing:14742
Prusa Logo http://www.thingiverse.com/thing:255639