As one of Yammer's first paying customers (albeit not in HE) I have quite a lot of experience with it. In general, it certainly appears to be the best of this growing group of internal micro-blogging tools. It's free for any size of network (as long as you don't need the additional security features).
I saw our network grow very, very fast organically – but there comes a point where it has to 'cross the chasm' in order to become a valuable business tool for everybody. And that's where the rubber hits the road, because – just like any other tool, system or process – it then needs management support, proper communication, use cases, and people who look after the community that it creates.
So, my take would be: trial it (it won't cost you anything), but don't expect it to have any meaningful impact without further thinking, resource and investment behind it.