We've used Yammer inside our small company, but quickly gave up on it because we are few and I think Yammer is a lot more useful in big companies were you should keep up to date on what the other groups around you are doing.

The implementation is pretty solid though and I liked the features provided. It really is Twitter for the Enterprise, so this acquisition makes sense for Microsoft.

Some smart people like Coda Hale also work there. I wonder if they'll leave or not.