KBFX is a replacement for KMenu, the default KDE menu. It's a WinXP-like menu, but it's a quite improved. It's possible to define themes for it, which can be downloaded at Kde-Look site, besides define a image as a start button. Its navigability is easier than WinXP button too, the items are showed in categories, what facilitate the users to find out their favorite softwares. The KBFX's configuration panel is easy and intuitive, and the themes can be easily added.
On the other hand, there is, in my opinion, a more improved KMenu replacement than KBFX, the first post of this blog say about it: Kickoff, a KMenu replacement created by the KDE and usability team at SUSE. Kickoff is quite special because it has more integration with KDE environment, for example, it's easily possible to add a favorite software or folder in Kickoff favorite menu just dragging and dropping a item on it, besides it, you can replace items in Kickoff menu.
In default configuration Kickoff menu is separated in five tabs: My Favorites, Recently Used, My Computer, All Programs and Leave, and each of them has a well divided and easy-to-navigate items. Specially in the Computer tab, the removable devices (USB devices, for example) are dynamically added when the user put a device. The real-time search rocks, it uses the beagle index to find out items in the computer.
So, KBFX and Kickoff are very good projects, but Kickoff is quite completed and integrated, otherwise, KBFX has the theme configuration, a real good functionality.
So, choose your menu!