Add Julia to the mix as well (which I currently use and I find personally better than those other ones).

I think TensorFlow's niche would be in the area of prototyping new ML algorithms as it seems pretty general, flexible, and fast. If you just want a simple deep neural net, it might be better to use Caffe or Theano. Those do not provide a flexible and general optimization framework, though. TensorFlow also seems more powerful in the area of language processing, as you'd expect.