Been using for years. It actually hides sold out sizes from customer view. Keeps them from calling and asking "when are you getting size L in again?". Really love this app. If we get inventory in size L in again (or a size L return) it automatically appears on the Shopify store without us doing anything.
If you are looking to manage all those broken link 404 errors that Google is reporting, the this is a great App. Saves a lot of time by automatically creating redirects for products with no stock and deleted. I would recommend.