To long-time Objective-C developers – especially those with an interest in modern programming languages – Swift is a very welcome and exciting step forward. At the same time, it can be frustrating at times due to the (current) state of the developer tools.
This is from Duolingo's thorough and fair assessment of the pros and cons of building a production app in Swift. I myself have been burned too many times by Swift in recent months (having to rewrite classes in Objective-C when things didn't go as planned) to consider it production-ready. But I'm glad to see that Duolingo is having success with it.