Viewing/Releasing Mails ist not very complex for end users. We have 200 users in our company who are using the Postini Spam Frontend. I have no reports that some had a problem with this frontend.
From Postini, the users get every day a mail which lists all new mails which are marked as Spam.
It has 4 colums:
Subject (linked to the Postini website to view the message - you have to login to see the message),
Date and a
"Send/Unjunk" link for each spam mail (directly unjunks mail without login).
The benefit of viewing/releasing on a frontend can also trigger spamassasin to learn what is HAM and SPAM.
The idea with the Junk folder is nice, but normal users don't look into that folder regulary, but they look into the spam/quarantine report mail.