It works, but it's clunky!
Just installed this on my dev SP2013 on premises environment.
For some reason it wants to re-login to Yammer every time you hit the page containing the AppPart. Plus it wants to re-login everytime you change your AppPart Yammer settings (surely this should be up to the page owner to point at a particular Network/Group/etc) and then re-login again after saving the settings.
Might as well have just used an iFrame to the Yammer.com site...
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