I just sync'd my 230 users last night.
Try that with one user, it worked great for me. The only thing I had to do is make sure that clear text logins were usable instead of using ssl. One nthing to note, I had issues until I added the --noauthmd5 bit to the end of the string.
imapsync --buffersize 8192000 --nosyncacls --subscribe --syncinternaldates --host1 old.server.com --user1 usernamehere --password1 passwordhere --host2 new.server.com --user2 usernamehere --password2 passwordhere --noauthmd5
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now