Well I grabbed this app. I temporarily had an ipad I purchased specifically when I heard about this app as I wasn't aware the ipad did text to speech previously. It cost me a whole 2 bucks but you have to pay more for additional foreign language voices.
Regarding text to speech sound quality I found it was excellent. The interface is pretty nice too and it's nice and easy to add books from the Gutenberg project. I grabbed the Spanish voice and was listening to Don Quijote in Spanish and it's pretty good. I didn't see any other Spanish titles which is what I'm mostly reading these days. The English selection is obviously darn good in Project Gutenberg although I'm sure most people like me would rather add their own ebooks from their collection which as far as i know isn't possible now.
The reader simulates the book reading experience showing you book graphics even simulating the sound of turning a page. I couldn't find a way to turn this off - I don't want to listen to the pages turning but it's just a minor annoyance I can live with. Also, as far as I could tell the app wouldn't function if you turned the screen off to save batteries which is annoying. For the ipod version I like just putting the ipod in my pocket while listening to the book and not necessarily reading along with the screen. I like the option to read along however. Especially in Spanish when there's bound to be vocab I'm not familiar with.
A newer version I'm told is going to support epub books and also do dictionary lookups for words including foreign languages. I'm really excited about the new version. When I got the app and was emailing the development team with questions (someone had time to reply to me very promptly even on the weekend which I was shocked about ) and was told the new version will be out very soon. I'm holding my breath heh. oh well. That was about two weeks ago.
Regarding similarity to apple's stock text to speech - I tried that briefly and it was a nightmare for me. Not necessarily the quality of the voice but the fact that I had to switch my ipad into handicapped mode. I could barely navigate the device with it on and it was talking to me constantly. It was a nightmare.