With due respect to the previous posters, Text to Speech is very valuable to me.
I bought my first ereader, the Astak Pocket Pro because it had TTS. Unfortunately the quality was terrible.
Though I bought a Sony to have more updated specs, I am pleased that the Onyx that I won in the last contest does have TTS. And I have used it. It is comparable to the TTS that I have as a standalone software I bought back in the Win XP days (didn't work on Vista, but does work again on Win 7).
Also Onyx is planning to upgrade their TTS with a new voice which should be an improvement over their current one (which is acceptable)
My original thought was that I could listen to the ebook with TTS and not have to convert it to an audio MP3. That takes time and space and if I didn't want to listen to the book again I wouldn't have the excess baggage.
My eyesight is deteriorating such that I have difficulty reading even on an eink, so I prefer audio.
Also, I use my audio books to lull myself to sleep (in lieu of the music that some people use)
Keeping a book playing in my ear has kept me sane sometimes
AJ