(Btw, just "Ogg" without the vorbis denotes a more general codec
container format. The Xiph folks a developing a speech and video codec
as well, and you might have heard about "Ogg Speex" and "Ogg Theora",
two other codecs sharing the same wrapper with Vorbis.)
How does Ogg Vorbis compare with mp3?
It has a vastly better sound quality at low bitrates.
If you don't care about quality, you can get much higher compression
while the sound is still recognizable.
It is free, mp3 is not. mp3 decoders are free for all, but writing
an encoder requires licence fees.
It is slightly more cpu-intensive to encode and decode than mp3, and
does not work as well on architectures without floating-point math
(such as the ARM CPU used in many organizers and gadgets), but this
is being constantly improved upon.