I use a converter called nsf2midi, and like most anything2anything converters, it's not perfect. You can playback in all of the different midi patches through the program but when you log the midi file the patches all reset to Grand Piano, so you'd have to do some 'patchwork' after the fact in another program that can edit midis.
Any time it detects noise channel it uses a hi-hat, and dpcm samples are a bass drum hit. Also, the triangle channel is not recorded. And pretty much every expansion besides VRC6 also isn't recorded. So, really, not the ideal converter, and I'd recommend sticking with whatever else you've got. I'm just really comfortable working in famitracker and I haven't learned how to make midis any other way, so when an OHC springs up I go for this convoluted method.
Anyway, here's a link to the source program:
http://www.zophar.net/utilities/converters/nsf2midi.html