Ask HN: Optional Types - How useful are they? (Dart)

I was reading about the optional types in Dart[1] and they make a lot of sense to me particularly as they provide another way of documenting your code. Since I haven't actually used Dart, or another language with optional types, my question is how useful are they? Also do they have any serious disadvantages? [1] http://www.dartlang.org/articles/optional-types/