The interface, if not a little ugly, gives you a quick outlook of all your transfers, sites and disks and makes it easy to drag and transfer. If you're looking for a solid FTP client what more could you ask more? In fact, FileZilla is not just limited to FTP, but also handles FTPS and SFTP for secure transfers of multiple files on multiple servers. It's popular, works well and fast, and regularly updated.
You could pretty much consider FileZilla an open source success story.
