Hey Caine. I've been following that other branch with interest, but I didn't want to annoy everyone there by asking you about Yammer. I work there so I'm clearly biased.
I just want to know what you like about Yammer and what keeps you from using it for more things. Why is hipchat more appealing than the Yammer chat? Internally, we use Yammer for almost all the things you listed here, with the exception of tasks. I don't think Yammer covers everything, but I'm always curious to hear about workflows that it doesn't support well.
Appreciate any feedback.