I have an older Vocaloid that I’ve used a bit. And Papi is right, it is very cumbersome to use. One of the key problems is that it was initially developed for the sounds used in Japanese speech. They later added English phonemes and you have to enter the text phonetically. However they way they spelled the phonemes was different from what I could find in any dictionary’s pronunciation entry. So there was a lot of trial and error trying to get the right sounds. I think they have improved this (somewhat?) in the latest version.
There is this available too. I haven’t tried the demo.
http://www.virsyn.de/en/E_Products/E_CANTOR/e_cantor.html