I have an X4 with vocalizer, that I use together with a midi controller keyboard. I like a lot the compact internal vocalizer, although my external Digitech Vocalist Workstation EX produces slightly cleaner vocals and a greater number of configuration possibilities. For my needs, and my quility demands, the internal Ketron vocalizer is quite enough. An external high end harmonizer like the TC Eletronics will offer you better results, of course, but at quite different budgets...

-- José.