I use it and love it. I mainly use it for frontend stuff to replace most of the javascript. Writing code in Dart is easy, fun and PREDICTABLE.

It also force me to write frontend code in a more engineered way because that's how it works, which benefits myself down the road for the millions of times I revisit and update the code.

My only problem for Dart is that its doc is not complete and dart:js is lacking comprehensive example for complex library.

But all in all I enjoy Dart a lot for my frontend development.