I play in a band where I have to do almost everything. The keys, the bass, drums. The only thing I can't do is sing. This doesn't make me sing, but I can do a lot now. I feed it a simple synth sound, then I have a vocoder that is clearer than any I've ever tried on (modern) famous small keyboards. I've been using it to emulate synth parts, horns, hit some harmonies under some high notes to give the poor singer a lift :).
Very versatile, and potentially if you set it up and feed it well, a great sound.
There is a great demo on youtube, if you search for "Electro Harmonix v256 Keyboard Demo" , by James Bowers.
What I did not like. The power feels somewhat flimsy, and it is very easy for the knobs to be changed by accident, if you're not used to using things like this as a keys player. Also, I can't say that it's the most easy to understand the operation. It's not very logical, for me anyway. I got there though!
No need to use the instrument output, the vocoded signal is in the mic one!