I've been using FTP Synchronizer for awhile and generally have had pretty good results with it. But, I've just moved to a Mac full-time (at work as well as home now) so want to get a native client if I can.
I've tried the only one that I've found - SuperFlexibleSynchronizer - but it crashed every time I loaded up an FTP to FTP synch attempt.
The most important features to me are: 1) ability to synch with a large number of files (thousands), as I generally work on sites with large number of files. 2) FTP to FTP synch. This would be very helpful as I work with some CMS based sites for which users upload files while on staging and don't want to move files locally first before moving live.