After email and web browsing, I think FTP is my next most common computer activity. I develop websites so having access to a solid, feature-rich FTP client is a must. I have tried several FTP clients on both Linux and MS Windows and I have to say that my favorite is Konqueror.
Konqueror is KDE's default web and file browser, but you can open two Konqueror windows - one displaying your local files, the other displaying your remote files - and just drag and drop. This also allows you to open both local and remote files from the same application. You can bookmark your remote FTP location and have your usernames and passwords handled by KDE. As far as I know Konq automatically detects if it should use ASCII or Binary transfer; I honestly haven't been able to find how to configure this with Konq, but after what must have been several thousand files transfered through Konq, not a single one has been corrupted.
Kbear and Kasablanca are two very popular GUI FTP clients for KDE. I've used them both and have found them to be more than adequate. I had some stability issues (this was a couple of years ago) with Kasablanca; it seemed to crash a lot. This could have been caused by any number of things, not necessarily their fault.
Before I started using two Konqueror windows, I tried a Firefox extension called FireFTP. Because most of my FTPing was related to my web development stuff, having an FTP client tied to my web browser made a lot of sense. It was fast and had all the features I needed. I had some problems with some buttons and other UI elements disappearing, but this was in its early stages of developemnt, I haven't used it in some time, but I'm guessing many of these issues have been resolved.
gFTP is an FTP client commonly used with GNOME desktops. I haven't spent much time with this client, but it seemed adequate and I have read plenty of rave reviews on the internet from satisfied users.
Popular CLI FTP Clients: FTP, Wget