Simultaneous logins for users w/ network homes

Hi. XServe G5, 10.4.7 running as Open Directory Master. Clients' AFP homes are on the server and work well. Clients are eMacs, OS X 10.4.7 and 10.4.8. I'm trying to use Fast User Switching... and it just won't work anymore. For some reason, it used to work fine for some users a few weeks ago, while others trying to logon would just receive the standard "cannot login, home is on AFP/SMB server" error message. Now everyone has the issue. I heard that NFS homes don't have this problem. Should I switch everything over? What will I lose? Is Leopard Server supposed to fix this?