I feel like Firebase has a great hackathon demo, but still find it hard to believe that people might actually be using this for their actual backend. The lock-in is extremely high, and migrating to something else would be extremely challenging.
For instance, we use Firebase for our community feed on http://spotpot.co but I would not use it to store user accounts there. In the end it is always a trade off but consider that for heavy real time applications a service like firebase can not only be convenient but also save you load on your infrastructure.
I'm writing it with om/clojurescript; however, the same would be true for reactjs or any declarative setup. The only code I would need to rewrite would be the read and write hookups to the server.