TTS for Semitic languages

I was discussing TTS for Semitic languages with the WAI group the other 
day, and we had some general questions that I was hoping people with 
experience in the area might have be able to offer some input.

The questions center on experiences when rendering text with versus 
without vowels.  A human would most likely not have problems reading 
something written whether or not it contained any vowels (assuming the 
person spoke the language).  How does a TTS engine do though?  Is the 
output reasonable when no vowels are present in the input?  I'm not sure 
how to qualify reasonable - either "the output is understandable", or 
possibly something more quantitative, possibly "60% of the words are 
pronounced correctly".

Thanks,
Jeff