It's not as easy to use as it could be, you have to do some work to get the words in e.g. there's no "I" so you have to construct the diphthong from "Ah Ee" in combination - something that could be made way easier to use if Realivox had a 'user dictionary' feature where you could store the syllables for frequently used words - but it's very flexible.
East West do a choir instrument that can do similar, but it's got a very different classical feel. I'm sure there are others around too I'm not aware of, but it's a bad example of something to knock IMO.
Having my PC swear at me in perfect pitch is good puerile fun. Takes me back to the days when I could get my Soundblaster to sing "What Shall We Do With The Drunken Sailor", if any of you are as old as me