I feel as though Roland had the human factor in mind from the very beginning. The idea that people would actually be using their instruments as tools to accomplish a task. But, of course you've probably just about heard enough of my Roland propaganda. I'll admit that I myself might be growing slightly weary of it. Anyway, the Roland VP-330 Vocoder Plus is a really great instrument. A friend of mine has one and I've used it a little on some recordings. Now, I'll say right now: I think vocoders are ridiculous. I'm sure you could do great things with them, and great things have probably been done without my knowing, but I've never heard something that I knew was a vocoder and thought "that sounds great!", mostly upon hearing a recognizable vocoder I think "That sounds awful!".
[The Human Voice section of the Mk-II version of the VP-330]
Mostly what's great about this instrument is it's human voice sounds (made famous by Vangelis), which can be used as a carrier signal or independently from the vocoder. I prefer to use them independently, but if ever a vocoder were to sound nice it would surely be with these sounds. They actually sound like human voices. Some how Roland made analog circuitry sound like believable human voices. No one knows how. It's a miracle! There's not much else I can say about it, but do read Roland's original advertisement for it (above). Also note that, if you factor inflation, the current going price isn't that different than the original price.