>For those that are leveraging both systems, have you integrated them to create a more robust KM tool?

We use a Yammer web part on SharePoint sites to display feeds, usually from Yammer groups.

>If so, do your users like it?

They like it.  We would like to be able to integrate other types of feeds (e.g., topic pages), but so far, that is not available.

>How tight is the integration?

It's pretty good.  The web part allows users to view all group threads, join the group, and post and reply from SharePoint without having to leave the SharePoint.

>Does search span the 2 systems or is it still separate?

Not currently, although this may be possible in the future.  For now, when doing an enterprise search from SharePoint, on the results page next to the search box is a link to "Try this search on Yammer."