Rough estimate is the Outlook app has over 70k lines of Swift, which is just over 50% of our codebase (the rest is Objective-C). That doesn't include external l

Rough estimate is the Outlook app has over 70k lines of Swift, which is just over 50% of our codebase (the rest is Objective-C). That doesn't include external libraries. One of our devs starting adding Swift code to the project about a year and a half ago. After that proved successful, the rest of the dev team then started getting up to speed. For the last 8 months, all development has been done in Swift. We don't arbitrarily rewrite existing source, but if we're making changes to an existing Objective-C file, we evaluate if it would be worthwhile to rewrite in Swift. Often the answer is yes. From our perspective, Swift is absolutely ready for primetime. For a large codebase like ours, it takes some time before you see direct benefits, but if you keep with it, I'm sure you won't regret it. I should mention the downsides: the migration to Swift 2 took time, but it wasn't awful. I'm sure we'll have similar issues moving to Swift 3. Also our compilation times are quite a bit slower than with purely Objective-C.