I've been playing with Timidity, a software MIDI synthesiser that uses GUS patches, and I've been really impressed with the results compared to my soundcard's usual MIDI synthesis. I've made some Ogg Vorbis files of Timidity's output and uploaded them, along with the original midi files. This isn't anything close to being a scientific comparison, but it does show what Timidity is capable of across a number of different styles of music.