I know this as already been discussed, like there :
http://forums.autodesk.com/t5/design-and-documentation/transparency-of-bodies-when-designing/td-p/52...
But the provided solution is not simple at all.
On complex models you have objects getting in your way. You can hide them easily (bulb in the browser) but not making them semi transparent. Modifiing the appearence is ok but when you want to to this 50 times a day it's quite a hassle to get back in appearence each time we want to change this. And we need to be able to see through some pieces with the geometry of other peices.
It woud be easier to have an item in the browser menu (under show/hide) and a submenu with default values to select: 10% to 90% by increments of 10 % would be far than enough i think.
It coud be also be in the right click ring on a body (wich is under exploited in my opinion)...
Thx autodesk team...