We've moved over to Tensorflow from Theano around a year ago. I'm a Software Engineer on the team and here's what I think are advantages from my POV:
1) Transition was fairly straightforward, both APIs' interfaces are more-or-less similar and share some design characteristics.
2) Having said that, TF's API is easier to use and without a doubt a lot easier to read.
3) Consistency: Deploying Theano in different environments surprised me on several occasions with different output compared to the training environment. TF is more consistent on this front (never had such issues).
4) Running multiprocessing with Theano + GPU is a disaster (due to forking) so I end up having to create process pools before initializing Theano. No such issues with TF.
5) TF provides many helpful operators (such as queues and batching ops) as well as monitoring tools (Tensorboard) and debugging tools.
6) Its development is extremely rapid, new releases every couple of months with a lot of improvements and new features every time.
In short, TF is what Theano should have been. A lot of new papers are being developed in TF as well so it helps to understand it.