I have experience with Plesk 8 (never tried lower versions before).
It will move the domain information, and create the necessary FTP login to match, and the password will work.
It will move the web data
It will move databases, but it will prefix them and the usernames for them, so none of your scripts will work anyway, until you fix the references in them to connect to said database
It will move mail accounts, but the expereince with mail in those accounts is mixed. As are passwords for those accounts. You may find yourself spending some time resetting all the mail passwords when users discover they can no longer get in.... and fielding complaints when they ask where their saved email went.
Mailing lists are a toss... it sometimes worked for me, it sometimes did not. There is plenty if sites on Google that can walk you through the process of migrating mailing lists between 2 servers though (settings and everything), so you might be better off going with that route.
Subdomains are a no go. Plesk stores their subdomain data completely differently than cPanel, and it simply will not happen. cPanel will not even recognise that there are subdomains in existance. For these, you will have to manually recreate, and FTP the data over.
Paths are different between cPanel and Plesk... Plesk uses /home/httpd/vhosts/domain/httpdocs whereas cPanel simplifies it to use standard Unix format of /home/username/public_html so any references to paths will need to be redone.
Essentially, expect at least a full day of downtime, unless you keep the Plesk server going while you reconfigure everything migrated to cPanel.
My suggestion would be to take it a few at a time. Use the built-in Migration manager for WHM to move the primary stuff over. Then go and set those ones up, taking into account the above notes. Once they are ready, move over the next batch. This will keep things simpler for both you, and the server itself (as opposed to having to reconfigure 500 domains at once).
Make sure you have a helpdesk available where users can log any bugs or missing things. Plan on having the Plesk server around for a month after the migration, just incase you need to retrieve something that the Migration Manager missed. Warn your customers that after 1 month, anything they find missing will have to be replaced with a new alternative.
Or, and this is a BIG "if" statement... now that cPanel is rolling out v11 in the Edge series, they may actually start updating the Migration Manager, which is something they said they were going to do over 6 months ago. But I doubt it will be done before it's time to take the trash to the curb for pickup (read:Plesk).
