I've been porting a larger application from JS to Dart, and I can repeat a lot of what Seth says in this article. The experience has been generally very good. While porting I also discovered a couple bugs that existed in the original code base for a very long time.
I do keep flip-flopping on whether I'm going to seriously commit to it though. I really like Dart, my code is safer, cleaner, and more maintainable. But it just doesn't feel like it has much momentum yet.
I'm not sure what I can say about Dart adoption internal to Google, so I'll be conservative and say that it's going well in my opinion. I hope we can speak more about it at some point. Language adoption is a very gradual thing at first, so I'm not at all surprised that momentum isn't that apparent yet. The community is very active though (join the email lists or G+ if you're not already).
I do keep flip-flopping on whether I'm going to seriously commit to it though. I really like Dart, my code is safer, cleaner, and more maintainable. But it just doesn't feel like it has much momentum yet.