The Theta TTS has good quality voices and you can purchase an individual voice for a very reasonable price. I've found the version I've used to develop this server to be very stable. This was version 2.5.0. I've had some stability problems with version 2.5.3, but don't know if this is a result of the software or some subtle bug with the latest gcc compiler (version 3.3).
There are actually two servers in the tar ball. The first server is a very basic single voice server called theta and uses the tcltheta.so shared library for the Tcl bindings. The second server is more experimental and uses Cepstral Theta's built in support for SSML to support multiple voices and changes in voice properties, such as pitch, volume etc. The support for SSML in Cepstral Theta is described by Cepstral as a work in progress. The same could be said for my server, which is called theta-ssml and uses the tclthetassml.so shared library for Tcl bindings.
Anyone thinking of writing their own server for Emacspeak may find the theta server interesting and possibly useful. The single voice theta server uses generic-voices.el, which essentially just outputs very simple text. It would be very easy to use generic-voices.el with any basic speech synthesizer, either hardware based or software based. I plan to use the generic-voice.el file with a script I'm developing to create an updated server for the DoubleTalk QT hardware synthesizer. I'd also like to do the same for flite and festsival.
Although there are some servers avaialble for flite and festival, I feel they have not been integrated into Emacspeak in the same way as other servers which are packaged with Emacspeak. It is therefore unlikely these existing servers will ever become part of the standard Emacspeak package. This is unfortunate as it means the installation of these additional servers is more complex than it needs to be and there is a chance not as many potential users will take advantage of them as would otherwise be the case.
The tar ball for the Cepsstral drivers includes instructions on how to install the system. This version has been made compatible with Emacspeak version 20. If you have an earlier or later version, you may need to modify things a bit or int he worst case, patch things. For version 20 of Emacspeak, it should be pretty straight forward.
If you find problems, bugs etc and I'm sure there are plenty, let me know via the emacspeak mailing list. I don't have much spare time to work on this at the moment, but will try to respond to all posts. This stuff is definately a work in progress, so its probably not for the faint harted.