there is no sound quality difference assuming no plguins are used.
what it does offer is
Open component architecture allowing third-party developers to extend functionality of the player
Audio formats supported "out-of-the-box": WAV, AIFF, VOC, AU, SND, Ogg Vorbis, MPC, MP2, MP3, MPEG-4 AAC
Audio formats supported through official addons: FLAC, OggFLAC, Monkey's Audio, WavPack, Speex, CDDA, TFMX, SPC, various MOD types; extraction on-the-fly from RAR, 7-ZIP & ZIP archives
Full Unicode support on Windows NT
ReplayGain support
Low memory footprint, efficient handling of really large playlists
Advanced file info processing capabilities (generic file info box and masstagger)
Highly customizable playlist display
Customizable keyboard shortcuts
Most of standard components are opensourced under BSD license (source included with the SDK)
<taken straight from foobar2000.org>
so in the end, its kinda like IE vs firefox. i have been using firefox for quite some time and appreciate its tab browsing ability and all, but still fail to see whats soooo great about it. the thing is its just marginally better in every aspect and doesnt have any glaring drawbacks. IE is still needed however as quite a few sites are designed for IE only and wont work properly with firefox (stupid web designers, oh well) foobar2k vs winamp appears to be a similar case where winamp has nothing to offer over foobar2k. the only reason i could imagine someone preferring winamp is for its milkdrop visualization plugins. i usually do other stuff while listening to music so this doesnt pertain me.