These answers are correct: it is a bitch to install (and keep plugged in even with the zip ties) and on a good HDD you may see up to 10 MBs. It also blocks use of other devices.
My MkIII has never come loose...it's almost as if it's glued in - and I didn't need to use the tie-wrap that came with it either. It needed a lot of force to get it seated correctly into the rom sockets though (the pins may be round not flat - hence a tighter fit than the flat pins on rom chips...but I can't be 100% sure about that as it was some years ago now).
If you don't push it all the way in it will try and pop out. Maybe they have flat pins now though so that would definitely make fitting easier.
