If you mean by 'credible' whether the school operates then yes, I know they do because I took my TEFL course with ITTP Prague in August of this year.
Since completing the course I have been teaching English in a lesser conservative region of the Middle East and my certificate earnt at ITTP Prague was well-accepted at the university where I now teach.
As far as I'm aware the rest of our TEFL study group stayed on in Prague to teach and I'm the only one from our group who left to teach in another country but I did sign up to their Facebook Group page and I'm in contact with some of their graduates in other places around the world.
I'd recommend ITTP Prague but then again I can't really compare as this was my only TEFL course taken.