Maxum Rumpus
The problem I had with the FTP server built-in to OSX Server is the need to create full-fledged OSX user identities for ftp access. I ran a server with over 450 ftp client identities, and didn't want to handle that size of user database.
Granted, that was back in the days of 10.3 and 10.4 server, so maybe it's a little easier in Leopard.
Using Rumpus, I was able to create the user list by scripting the export of a tab-delimited text file, output from a mysql database via php scripts. Used to impress my wife with that one.
Not quite sure if the 10-client license for OSXS would limit the number of active ftp accounts available either.