Atomic - 6:48 pm on Jul 15, 2011 (gmt 0)
I've made changes to a 15 year old site in the last 7 or so months and seen traffic nearly double. Every single page on the site saw substantial changes. One page in particular has gone from page 15 to #5 on page one after changing the layout and some of the content.
I would do it again in a heartbeat.