Doug Mahugh and a bunch of the standards crew (both in and out of Microsoft) have been having a great discussion on document format interoperability. They (and referenced posts) are worth reading.
Michael Kiselman has been publishing some great case studies on the use of Open XML. These cases studies are hard evidence about the uptake of the format. I really love seeing people putting the document formats to good use. Folks are building innovative solutions that really wouldn’t be possible without using Open XML.
Microsoft has a group (The Microsoft IT Business Intelligence Center of Excellence Core Scorecard Team) who delivers key information to more than 3000 people worldwide, sometimes in the form of automatically generated macro-enabled spreadsheets. They wanted to deliver these through Excel Services in SharePoint, but SharePoint doesn’t allow web delivery of macro enabled spreadsheets. Well, they did something cool – before delivering from SharePoint to the user, they add (using Open XML) digitally signed VBA code that contains the necessary macros. This is an interesting read.
Darwin Information Typing Architecture (DITA) is an XML based solution for topic-based authoring. Content Technologies has built DITA Exchange, which uses some of the cool features of Open XML to allow people to author DITA XML in Word. They translate DITA XML to Open XML and back again.
Savo Group provides an on-demand collaborative Sales Enablement solution called SAVO. Customers wanted documents delivered in Open XML. Using the Open XML SDK, SAVO generates customized documents for download to end users.
Look for these among the studies that you find here.