Same problem!
I am just testing a new site prior to launch on 2.3 (Build: 20120625) and have found the same problem.
If a user is enrolled into a course with ANY role then they cannot see the course report, only their own completion status. If I enter the course as admin, without being enrolled, I can see the report okay.
I've checked the role assignments for report/completion:view and it is set to Allow for all roles from non-editing teach upwards.
Not sure what I am missing but it's driving me nuts!
Mike
UPDATE: As a bit of a test, I took the Course Completion block files from a 2.2 installation where it is working fine and replaced the 2.3 block. The error persists, so I don't think it is a problem with the block, rather the capabilities...