Failed miserably in 10.5.4 under Mail 3.4 on a test copy of my Mail folder on a Tester user account. It rebuilt the Envelope Index but with no information in it. All 7318 messages are unread, have no titles, and were received at the time of rebuild. Not to mention that there are no message contents to be seen when selecting one. All the .emlx files are there but none show any contents when opened directly (double click the file) into Mail.