According to an article posted on December 11, 2006 on BusinessWeek.com (and now if this link expires you know how to find it), worldwide sales of Filofaxes have gone up 20% since Letts acquired the company in 2001. Growth is currently at a clip of 5% per year.
The article talks how Letts has endeavored to make Filofaxes more appealing to women customers (who now make up 60% of Filofax purchasers, as opposed to 60% men in the mid-90's), and in developing countries, where Filofaxes are desired as a status item.
Why else has the Filofax held up so well against Blackberries and PDAs? According to the article, people use electronic devices more for "wireless connectivity than as true organizers."
Another interesting statistic: 68% of Filofax users are under the age of 45.