I used iTunes to convert it, and the resulting sounds were terrible, because iTunes has to decide which instruments to assign to each track (and the Quicktime Musical instruments software isn't the highest quality). If the composer of a particular MIDI track didn't assign instruments according to the General MIDI standard, then your results can be pretty random.
The classic way of doing this would be trying
MIDI to MP3 Converter. It converts files quickly and conveniently. And, the best part, you can choose from many sets of instruments (called soundfonts). Would recommend to everybody.