Mobile interfaces for client websites: worth it?

Having implemented a number of mobile specific interfaces for client websites, I wondered if they had actually been successful.

Mobile version of Caravan Sitefinder

So what would constitute success?

Basically, the main question would be:

Have mobile users been more engaged with a mobile version of the website?

Which analytics year-on-year statistics to use to see if the mobile version has been successful?

As a quick assessment visitor numbers can't really be a valid gauge as mobile traffic is rising so dramatically.

Mobile version of The Edinburgh Address

Average visit duration is probably out too, as a good mobile site will more quickly get users around your site and mobile visitors tend to be on a site for a shorter period.

Page views / visit

The one that seems to fit the bill is page views / visit.  If you are achieving a higher number of page views / visit you are broadly allowing users to navigate more easily through your content.

So, looking only at visitors using a mobile device, I took a recent month when the mobile site existed and the same period in a previous year when it did not exist and compared the two.

Here were the results:

Not really a surprising result, but it looks like users will be far more engaged with a specifically designed mobile interface than a traditional desktop one.