As a consultancy, we're using external groups with all of our clients. It's a great way to maintain a history of the project as consultants or client contacts roll on/off the project. No more forwarding old email chains!
For us, connecting our global regions together through Yammer has really helped break down some quite traditional silos around information and collaborating. We ran a rapid start with a client and utilised project teams all over the world to help deliver some creative solutions. http://www.atkinsglobal.com/en-gb/projects/heathrow-rapid-start. People were sharing video, images and ideas live from the client site and from all over the world on Yammer which was great to see.
Yammer is somewhat akin to Twitter in our org. A place to share insights with anyone in the company, or to engage in open discussions with anyone.
Just this week we had someone post a link to an article and a long detailed question about how this particular topic might impact our product positioning. The user added our CEO and some SMEs as a CC to the post. A couple hours later, the CEO replied and a deeper exchange on the topic was well underway.
We have some very active Communities of Interest in our Yammer Network. We have some very active office location groups, as well as popular groups related to specific technology interests (e.g. Mac users).
Yammer for us was to bring parts of the organisation that don’t always talk to each other (or interact much) together, sharing experiences and best practices, making staff feel more connected. That’s an ongoing journey but Yammer has certainly helped.