I finished migrating from two Sun Servers running Solaris 8 and CyrusSASL+Horde to a Linux Ubuntu 8.04 x64 Sunfire 4250 2xQuad Core Intel E5450, 14x146 GB RAID1-0, 32 GB RAM.
Migrated about 50k accounts with a external MTA+Spamassassin + External LDAP.
Working with a 200 simultaneous web conections and 90 IMAP/POP simultaneous.
All works smooth and quickly. I going to wait some months to delete the unused accounts because cyrus dont give me a log to check it!