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A lot of music players will be slow when starting if they have huge libraries, but CMus starts fast no matter the library size.
Pro Does not use a lot of memory
Cmus only uses about 15 MB. This is a very small and light media player, which is ideal for people with low end hardware.
Pro CLI only
Pro Customizable keybindings for a personal experience
You can add keybindings for just about anything - including seeking (forward/backwards 1 minute, for example) which isn't supported by all music players.
Pro Good library structure
A lot of music players may act more like playlist viewers rather than a music library. cmus sorts by Artist > Year > Album.
Pro Open source extensions and scripts
On GitHub you can find the official WIKI where there are a dozen extensions and scripts from color themes to a lyrics viewer and the ability to play YouTube songs.
Pro Works very well with tags from MusicBrainz database
cmus will factor in additional data from the MusicBrainz database while sorting. For example sort orders for arist or album, and the original release date for an album (in the case of a re-release).
Con Difficult to use
Cmus uses odd keyboard shortcuts such as "C" to pause, "E" to add songs to queue, and "4" to edit the queue.
Con CLI only
No graphical interface is available for cmus. Everything is done through CLI (command line interface).