I use a narrow style because I have the option without it being enabled for other users. I find that my monitor at work is too wide to read content in a forum without "head panning" and as a result I use the narrow style at work and the the full width one on my laptop.

The narrow style I use created has a couple of other "tweaks" such as larger icons for new posts in subforums, so that I actually notice that there is new content which is particularly useful on a large hi-res monitor.