I convert text to speech all the time, turning long documents I don't really want to wade through, essays I've written that need proof-reading, badly-formatted fanfiction, and so on, into audiobooks I can listen to on my iPod while doing other things. It's surprisingly useful as a proof-reading tool because the computer-voice will pronounce misspelled words as you've written them, pause for commas that shouldn't be there, and so on. I paste what I want into TextEdit, then use a simple Automator action to convert the contents of TextEdit to speech and import that file into iTunes. iTunes automatically sorts these files into my 'Run Bookmarking Script' smart playlist, so that I can find them to run the 'Make Bookmarkable' script from
Doug's Scripts to make them into proper iTunes-recognised audiobooks. The voice takes some getting used to, but on the whole it's great to be able to have your computer read you a story when you're too tired to read (or have a migraine), or to get through some boring article you're supposed to read by listening to it while you do something you like (me: needlework, or sudoku, or just hanging about in the fresh air). The only thing that irritates me is not being able to teach the system anything. There are words it invariably mispronounces ("drat", for instance, comes out as "doctor-at", at least in Tiger, and anything it doesn't recognise with "ver" in it comes out as "version"--try "in vino veritas" and you'll get "in vino version-it-as"), and it seems to me that it ought to be fairly simple for me to prescribe a correct pronunciation for those words.
I don't have reason to have the system read things to me automatically, though I have used that with some apps that can alert you with speech. For instance, I tried an application called Subscriber for a while, and its alert for a website having changed was a voice saying "A website has changed!" Kinda cool.
I've never been able to get either of my machines to understand my speech. With the iBook, it's because the microphone's broken. With the iMac, I suspect it has problems with anything besides a fairly standard TV-style American accent. I can sometimes get it to tell me the time, log out, or tell a joke, but not reliably. No amount of reading the training phrases seemed to improve the machine's recognition of my voice. I find this too irritating to bother with, especially since simple idiot cellphones seem to be able to handle voice dialing.