You get a very bright, "jingle jangle" type sound. The strings are double coursed, so the lower strings are doubled by an octave above, adding to the high end.
You get a much fuller sound from a 12 string guitar than you do from a 6 string guitar and surprisingly, it is not more difficult to play. The sound is much more rich and heavenly sounding.
You get a wider spectrum of tones, it will sound like a full scale. If all strings are played openly (not recommended) then it will sound absolutely terrible, played correctly, this instrument is great.