I think that fixed width is good on sites, but a fixed width of around 750-800 as this means it can fit on everyones screen (apart from those with old, small screens) - this also allows the layout to be the same on everyones screen and is easier to design. Although, 100% is OK for forums as there is usally alot of text on screen, people don't seem to mind as much in these cases. CMS websites annoy me when they use 100% width.. a box over here and another all the way over there with abit of text in the middle :S