Upgraded last night from 6.0.10 to 7.0.1 - CentOS 5.5 x64
Very smooth upgrade for me. 15 mailboxes. Mix of small (less than 20mb) and large (including 1.5GB, 1.3GB, 750MB, 500MB, 500MB)
FYI - I ran the pearl script to update the mysql tables immediately after the upgrade completed. It took 75 seconds for my install.
Note: If your users use the Calendar Daily Agenda printing facility you might want to hold off until there is a fix for this issue.