We're not using Dart commercially currently; though we are prototyping in it as a candidate for vNext of some very large applications. To me, it seems incredibly promising. Every time I have to go back and write JS in our production apps; I seriously miss the static analysis Dart does; it's so clunky to find trivial errors at runtime in a browser that the editor should've been able to show you!

However; there are some frustrating holes in it though that have made our prototyping tricky; such as:

Serialisation; not even JSON support. Alan Knight is working on a Serialisation library; but even that is frustrating to use (it doesn't support DateTimes at all well for ex), so I'm generating having to generate serialisation code from C#.

No private pub server. If you want to host Dart packages internal to your company locally; you're out of luck. Although the source for pub.dartlang.org is open; it's written to only work on AppEngine; which kinda sucks.

We're also slightly nervous that the Dart VM still isn't in Chrome. I don't know what's taking so long, but this would seriously help convince others that Google is really invested in Dart.