That incredible real human voice!
The Esperanto team deserves a LOT of praise for taking the trouble to record live, human-generated audio for the sentences in the course. I've studied three other languages with Duolingo, all of them using text-to-speech, and the difference is just beyond expression. It is so much more clear, so much more comprehensible, and infinitely more pleasant to listen to as you do the lessons. I wish German and French would get the same treatment! Hopefully it will be possible at some point to record the individual words--that's about the only shortcoming I can see, although it's a pretty minor one when you consider that you can always replay the entire sentence. Dankegon! Via kurso estas trezoro!
I don't have a problem with the TTS in most of the courses (I haven't tried them all, and I know there's some kind of A/B test going on with German; I also remember not feeling remotely confident I was catching any of the nuances in Danish when I dipped my toe in), but it has to be said that the EO voice is excellent. I think not just because it's a real person, but because the person who made the recordings is just very, very good.
If one could guarantee a voice that good in every language, I would most certainly vote for real person recordings for every language.