I like to convert my printed books and textbooks to audio books so I can listen to them while driving long distances to work. Few of the books I am interested in are available as audio books. I find it fantastic to listen to audio books since it stimulates the mind in some ways much better than by sight reading a book.
I tried GhostReader, TextSpeech Pro, Narrator, and numerous other applications.
Two key points are:
1. Is the audio output easily understandable like a real voice recording rather than a computerized voice? This means the sound has to be smooth and have prosody like real human speech.
2. Can the audio output be customized by speed, prosody, pronunciation, etc.
GhostReader is the best of the text to speech products because its output is much closer to real human speech than the others. The output can't be customized as much as with TextSpeech pro, however. But it wins out based on how smooth and understandable the speech is.
TextSpeech Pro is a very close second. It has the advantage of being able to customize pronunciation - thus making it more understandable (e.g. "read" can be pronounced "reed" or "red" depending on the context and you can choose which one is best for the output. The main problem with TextSpeech Pro that the output sounds harsher, more tinny, and is not as smooth as with GhostReader. The difference is very small. But I would regard the output from TextSpeech Pro as a B- grade and GhostReader as a solid B grade. Neither one is a B+ or A grade.
In regard to Cepstral voices and other additional voices versus the high quality built-in voices of Mac OS X Lion, I say the best for audiobook conversion are the built-in Alex and the Cepstral David. Alex is A+. Cepstral David is a solid B. The other voices are simply not worthwhile since the quality is too low - Cs and Ds for audiobook conversion.
TextSpeech Pro does have a complicated interface. I wish they would make it simpler. GhostReader's interface is ugly and old. But its output sounds smoother and clearer - just a little bit better than TextSpeech Pro. And its the output that ultimately counts.
TextSpeech Pro seems to be much more actively developed than GhostReader. I wish it's developer continued best wishes to improve the output. When it exceeds GhostREader, I will switch to it as my go-to tool.
I wish the Mac App store is soon update to match the version you can buy directly from the developer. The Mac App store version - which I have - is still stuck at version 3.5.x