I recently worked on a production app that used Swift. It's certainly possible, but I wouldn't recommend it (yet). Writing Swift is nicer and feels much more natural than Objective-C, but the tools are not there yet.
Xcode crashes often (even more frequently than with Objective-C), the compiler will probably also crash at some point during development. Compiling Swift code is also considerably slower than compiling Objective-C code. Error messages are often cryptic and debugging compiler bugs is very time consuming.
There's also a few places in Swift where things still need some more time to settle, framework support is tricky and some very basic tasks are harder than they should be (for example, getting a substring).
Xcode crashes often (even more frequently than with Objective-C), the compiler will probably also crash at some point during development. Compiling Swift code is also considerably slower than compiling Objective-C code. Error messages are often cryptic and debugging compiler bugs is very time consuming.
There's also a few places in Swift where things still need some more time to settle, framework support is tricky and some very basic tasks are harder than they should be (for example, getting a substring).