I have not found the canned airwave reportst to be helpful here either....
the client inventory report - and using OS Summary - seems to be pretty good - appears to line up with stats I'm getting from clearpass.
but you can also run queries against the underlying database:
select count(distinct mac),username from client_historical where connect_time > 1377993600 and connect_time < 1380585600 group by username
that will give you a count of mac addresses that have been associated with usernames within the timeframe.
It does pickup on everything.... ie even un-authed users etc... so might want to limit it by ssid/authstate etc....
I have not looked at linking in device type tables.... but could modify query to something similar - but group by device OS