I don't think I've ever received a comment from @tts on any of my posts, though I have seen them on other posts. When I first saw the service, I thought it was a great idea. I love listening to audio files, especially when I'm busy or driving, as there's so much rubbish on the radio! So I think it has potential, but I agree with you that the voice is monotonous...
I've just done a search for @tts. That account has produced one single blog post. The search revealed this exposé post about them!
https://steemit.com/spam/@themarkymark/the-dirty-little-secret-behind-the-tts-account