I too am interested in text to speech apps. I want to use it for Ebooks. So far the very best app I've found is called Voice Dream. It works well with PDF Ebooks, and a few other formats as well.
I'm using IPad third gen, and have tried the built-in auto-speak. It is not specially made for Ebooks or texts; it speaks everything you touch, and changes the way the iPad functions. I was not impressed.
While Voice Dream is excellent for the voices and reading, it is difficult to find PDF files of the books I'd like to hear, which aren't in public domain. I've purchased Ebooks on google, iTunes and Kindle for iPad, but none of these let me to transfer the file. Now that I have a great speech app, it's difficult to get the books.
Maybe this can be done very easily from a computer, but I use my iPad for everything.
I highly recommend Voice Dream though. I've tried the text reader on my actual Kindle, and it is not good at all. I think you'll be impressed with the Voice Dream app (in App Store). But let me know if you find a place to purchase Ebooks in PDF.