A few months ago, I wrote about my search for the perfect FTP client -- and how I didn't think it existed yet on OS X. I still don't. But I've recently purchased Transmit from the fine folks at Panic Software and have been giving it a spin.
It's fast, responsive and has some nice configurable toolbar options (shortcuts, previewing files, etc.). I still think that the one feature it's missing (and I've emailed a feature request to the company) is a "column view" for navigating server directories, much like that found in the OS X Finder. It's become such a natural way to view hierarchy for me, and with that feature, Panic would be the best FTP client in the history of FTP clients. For now, it seems to be the best FTP client in the history of FTP clients currently available for OS X.
My main complaint with RBrowser, which I had been using exclusively for a long time, was it's sluggishness -- and inability to transfer large, deep directories without freezing. Transmit wins on the performance meter -- hands down.
Also, the icon. It's one of the better OS X icons I've seen. Why? It's fat and chunky (easy to click) and looks cool when it's bouncing up and down (if you like that sort of thing happening in your Dock).