I have two of these, one I just bought (used-mint condition) and one I bought new several years ago which is now dead.  People who do computer-aided design and need fine cursor control absolutely love the FX because the large ball gives you just that - better fine control than any other mouse or trackball.  However, some actions, like drag and drop, are difficult to do with the traditional left button and ball, and instead require the use of the 3rd button as a drag lock, which is easy to do, but different than you may be used to -- once you get the hang of it, it becomes second nature.  The fourth button can be programmed for scrolling.  The buttons are programmable - you can choose what function you want them to perform.