aristotle - 12:57 pm on Aug 3, 2010 (gmt 0)
We monitored the search engine results closely for while. About 6 months in to the experiment, we checked our rankings and found that the page that had the article written for it was showing up as #1 when searching for its title, but the page that had no article written for it, but all else being equal, was showing up on the 4th page. This is a huge difference and verified our thoughts on article submissions.
I wonder if the kind of benefit you saw depends on where the article was published. If it was published on an authoritative site in the same niche, then I would expect a significant benefit. But if it was published on a general run-of-the-mill "article directory", I would be surprised if it produced much benefit.