How stable is Swift?
As with any new language, there is a potential for bugs. While you may encounter some trouble with the Swift language, the majority of issues were addressed before the 1.0 release.
The thing to most look out for is changes to the Swift language during each update. For example, when updating from 1.0 to 1.1, Apple introduced a new feature: failable initializers. You can expect that the language will change as more people use it and give feedback to Apple. Stay apprised of changes using the revision history for The Swift Programming Language.