I think MIDI is the best thing since sliced bread. MIDI, like anything else, is only as good as you want it to be. First thing, MIDI has no sounds, it is just a set of instructions. MIDI can be used to change patches on an effect unit, or ever change it's parameters.
What most people use it for is to send information to keyboards and/or sound modules, thereby producing sound. If you have a cheesy sound device, then you will have cheesy sound, but if you have a high end board, then your sound will shine.
The problem with some MIDI files is that people program the music in rather than playing it in. Having the notes without dynamics and human feel, will make songs sound cheap regardless of what kind of board you have. So to have a believable sound from your board, you should play it not program it.
Just playing your music to MIDI is not enough. You must play the sound as if it where the real instrument. If the sound is a fiddle, then play it's notes as if it were a fiddle, not as a piano. Playing a sound in a way that is different from the real instrument will make it sound fake and often cheesy. With this in mind, if you buy a guitar to MIDI converter, you cannot play keyboard sounds as if it were a guitar. Some sounds require longer sustains then others; your guitar skills will be hindered.
So what is the big to do about MIDI?
For recording purposes, I like MIDI for two reasons:
First, I can sync MIDI up with my audio; MIDI tracks require a fraction of hard drive space as audio tracks.
Second, is that I can edit MIDI tracks once they are recorded in ways audio cannot. You can add, delete, and modify MIDI notes from the recording, even if the note is in a chord. Hit a bad note? No problem, just move the data from the wrong note to the right one. Note too long or too short? Just increase or decrease the duration of the note. Note too loud or too soft? Just decrease or increase the note's velocity. How about during mixdown, you decide that a certain piano sound does not fit the mix. No problem, change the keyboard sound to one that does; you don't have to rerecord the track to change the sound.
