We completed migrating 2800+ accounts from iplanet 5.2 to zimbra this week using imapsync. It was a bit slow (10days for 2TB), but went fairly well. A full imapsync was run followed by additional syncs using the "maxage" option. A maxage of 5 with 2800 accounts finished in 4 hours.
The imapsync was staged on a third host and we used proxy accounts on sun and zimbra to access the end users account. Our config allowed for about 8 simultaneous imapsyncs.
After the account was fully converted into zimbra, we used Sun's ims "deliver" command to drop one final message in the old account, informing the user their email account now existed at a different host. This was important for pop and imap clients that still pointed to the old server.
Guide to imapsync - Zimbra :: Wiki