Transmit was already one of the very best FTP clients for the Mac, and it's only gotten better with its long-awaited leap to version 4.
The two most noticeable things about the new Transmit are faster speeds (especially when dealing with lots of smaller files) and a completely revamped, more Finder-like interface. Transmit's snappy, animated new interface makes workflows more natural and streamlined than ever. The file browsing has also gotten easier, with new features like "Places" (for storing shortcuts to frequently used folders) and useful Finder-style functionality (which has been showing up in other FTP clients). Transmit 4 is packed with smart new improvements, perhaps most notably Sync (a smart syncing method that lets you apply rules to files and even simulate a sync before pulling the trigger) and Transmit Disk, which lets you mount an FTP server right on your desktop--which you can see in the Finder (and access with other apps!) even when Transmit isn't running.
Transmit's performance, polish, and stability is unrivaled, and its interface sets a new bar for FTP clients. If you need an FTP client on the Mac, checking out Transmit is a must.