I've been using TTS for a few months now - the savings in time and voice over $ for Spanish translations of our courses (where using my voice isn't an option... I couldn't roll an R if my life depended on it) has been in the tens of thousands of dollars.
I use Neospeech - their voices, in my opinion, are orders of magnitude better than any free voice you'll find in any number of applications. TTS works for us because we create microlearning content almost exclusively - anything longer than 2-5 minutes or anything especially important we do a live voice over for.
Yes, we had to fight against change resistance - but there's been no negative effect on learning / retention. For us, TTS has been the right move, despite its limitations with the way it sounds. Long term these voices will only get better and cheaper - I believe that in our segment TTS is not only the fiscally sensible move, it's the correct move overall - we have had more time to drive the visually engaging aspects of our courses and have a more efficient workflow when it comes to corrections with the inevitable policy changes that occur in our business.
I for one am a huge fan of TTS and hope that Articulate continues to develop it towards with the end goal of giving us control over inflection and tone of a voice one day. I feel Articulate's voices are good but they lag behind some paid services out there.