I suppose it depends on your purpose really. For forums, I would say dynamic widths simply because I like to put my monitor to use. Forums look entirely different to someone with a 1920x1200 resolution than they do to a person with 1280x1024. I get annoyed with forums at a static width. Forum posts are mostly a few sentence's at best, so you can fit a lot of them in a screen height. For something like a blog post, I like narrower fixed width layouts because it keeps my attention. I suppose it's personal preference really. I use static widths in everything I do except for forums and commerce sites. My static widths generally stay at 600, 760 or 960 pixel widths.