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Foobar2000 has a clean, minimalistic UI, small filesize, and is light on resource usage.
Pro Functionality can be extended with components
Foobar200 has over a hundred components to choose from that add UI functionality, decoding support for various formats, etc.
Pro Plugins can be used to bypass Windows mixer to output a bit-exact signal
Through the use of a plugin (of which there are three popular ones), users can get bit-exact sound output straight to their DAC or soundcard. While not something every user will use, for audiophiles this is a pretty important feature.
Pro Supports a wide variety of audio formats
Foobar2000 natively supports MP1, MP2, MP3, MPC, AAC, WMA, Ogg Vorbis, FLAC, ALAC, WavPack, WAV, AIFF, AU, SND, CD, Speex, Opus, and support for additional formats can be added with plugins.
Pro ReplayGain support
ReplayGain can be applied on a per album or track basis, allowing the user to set up an even volumed listening experience.
Pro Advanced layout
The player has a layout-editing mode that allows for a wide variety of UI setups.
Pro Built-in support for Windows Media streaming
Foobar2000 can natively play ASX/MMS/RTSP streams.
Pro Variety of skins available
Browsing a site like DeviantArt will give you hundreds of skins (that have been made by other users) to choose from.
Pro Transcodes all supported audio formats
The converter component allows foobar2000 to convert from and to any file formats that the player is capable of playing back.
Pro Highly customisable
Foobar 2000 has powerful theming support.
Pro Internal search is a beast
Foobar2000 provides an extensive search to look through your library. It supports special queries to only look for certain tags and other conditions and is incredibly fast in doing so even with over 20k of songs in your library.
Pro MP3 player management
Users can easily send their music files to their MP3 device through the use of the right click menu when browsing their library.
Pro Good compatibility with Windows Media Center IR remote controls and wireless mini-keyboards
Foobar works well with Windows Media Center IR remote controls with virtual mouse functions; also with wireless mini-keyboards.
Con Not easy to skin
Out of the box it looks like something out of win2000. It is also very difficult to skin for the normal user.
Con The previous button doesn't work in random playback
You are unable to go back to a song when using random playback.
Con Locks files and folders, making it hard to manage them
Several parts of Foobar lock files and folders in a way that they cannot be file-managed (move/copy/rename/delete) easily in the program (and externally). With the waveform plugin it's even worse. Cannot delete a file until the waveform is fully scanned.
Con Cumbersome start-up
Foobar2000 requires a bit of work to get it up and running.
Con Breaks existing components every few updates
Makes it very hard to keep a customized version of Foobar running smoothly without worrying about non-working components.
Con Provides little in terms of hand-holding
Advanced tasks such as skinning and installing components may be a bit intimidating for beginners because they require knowing your way around the computer, not just the player.
Con Outdated Plugins
Many, many plugins are outdated and not maintained for years, leading to stability issues with a lof of them. A rather complex SDK and missing sufficient documentation makes maintaining or creating new plugins unnecessarily hard.
Getting to that neat GUI setup you've seen somewhere is probably impossible because of that.