Everything said in earlier posts is spot on. Each application for midi has its challenges, but great rewards also. I use an original version (1) Yamaha Motif keyboard synth, as well as a Yamaha ES Rack synth (primarily for use with my Carvin NS1 synth acoustic/electric classical (guitar being my main instrument). As my transcribing skills are limited (i.e., transcribing my own creations to sheet music), I use the midi thru from my synth to capture what I play in midi form (that provides the input for any decent notation software), and simultaneously capture the acoustic output from the guitar, and any audio sounds from the synth connected to my guitar. Very versatile.

The better the hardware (or software synth), the better the sounds generated (and of course, more expensive).

For a great example of what can be done with this setup, have a look a Steve Oliver playing these types of guitars with synth/audio capability.

Richard