The ADT approach does not replace full OO, and it's not a good excuse for Wolfram Research not having fully fledged vendor-supported OO, but it's certainly better than nothing, and may be a good alternative if you don't want to rely on the valiantly attempted user-contributed MTools package.
In fact, because it's Mathematica, and has powerful pattern matching, the ADT approach offers a lot that traditional OO coding language can't. And remember:
And, it's also easy to model it in Systems Modeling Language v1 (SysML®), very useful! A full slide tutorial trail about the recipe, including SysML examples of the recipe, is under development. There are already some Webel Policy notes with some code examples:
Visit also the comments by Leonid Shifirin on this forum posting on strong types in Mathematica.