I like the Cepstral voices. Nice to finally see a voice choice on the Mac. “David” and “Diane” seem to be the best (perhaps they are newer than the others?) with “Diane” being slightly more realistic than “David” (I think she is the newest of all). Voice synthesis technology continues to advance.

The voices range from so-so (the novelty voices) to fair (the British voices) to good. Its interesting, but each voice seems to be built from the ground up with its own speech engine, peculiarities, inflections and so on, not just the same engine with a different pitch applied to it like the built-in Apple voices.

I think a single voice is definitely worth $30 if you do a lot of listening or use them to make “audiobooks” to take on the road, but it would be nice if there were a discount if you buy multiple voices. One good voice is reallly all you need, though.

Cepstral has a well-designed site that allows you to test each voice, plus you can download the voices to test them out (but with annoying nag messages). One word of caution: the voices each run about 90 MB!