We use Yammer quite extensively.
We have hundred of groups set up now (big company) so it kind of loses its point then – I can’t get emails from all the groups I really need to keep my eye on – I’d just fill up my inbox in a day with notifications, so in that respect you actually have to go in and proactively look for things, which defeats some of the purpose.
When it works – small groups sharing files/conversations etc. it can be quite useful. Especially if you are over a number of sites.