People who are trained reading sheet music will be able to read ABC code in the usual keys (up to two flats or sharps), with reasonably little learning effort. The others are likely to rely translation to MIDI anyway.

Any sol-fa system is in a sense equivalent to notation in C major or A minor, which can be done with ABC as well. As I wrote before, this system has its drawbacks and idiosyncrasies, but now it's here to stay. I learned it while reading Mudcat threads and was reasonably successful with it before I ever took a look at the documentation or tutorial.

I would like to read more tunes posted in ABC notation again on Mudcat. Composers can post their own pieces as ABC code before they study and perform them, and ask for comments and advice.