FFmpeg addresses a rather small group of users due to its complexity and lack of a friendly graphical user interface. Although it is, in fact, an audio / video converter at its core, FFmpeg is far from the type of software most users are used to when it comes to converters.
For starters, since its inception back in 2000, FFmpeg has been constantly updated in order to meet the ever-growing expectations of the community and advanced users in particular. For those of you that may have heard about it, FFmpeg does sit behind various application software that only act as a GUI to the program, making it as easily as possible to work with this very interesting command prompt utility.
As long as you are not at all interested in software that just rely on its interface and less to none on the functionality, then FFmpeg can really pose a threat to the more popular applications that limit you on selecting from predefined profiles. On the other hand, many well-known programs use FFmpeg and this peculiar category comprises none other than MPlayer, VLC or HandBrake.
FFmpeg is surely tailored for command prompt control freaks that just happen to want complete authority of the whole process, that can spend stupendous periods of time tinkering with the settings and creating quite the number of profiles to serve various situations. Amongst the most widely-known container formats supported by FFmpeg, you may consider ASF, AVI, FLV, MKV, MPEG or OGG (OGV).
To sum it all up, FFmpeg is not the perfect tool for every user, but it can be considered to be the one tool to rule them all by a fair number of enthusiasts. Although its fanbase may not be able to compete with other ones, the members are, generally, more experienced and can really talk and spend hours on media conversion matters that others would not even be aware of.