Apple says that the feature is available for free and paid apps on all of its platforms, meaning that iOS, macOS and tvOS devices are all supported (as long as they're running the most up-to-date software). Also, Apple won't take your money until the day the app is released, similar to how Amazon handles hardware pre-orders via its online store.
If a developer decides to change the price of an app before it launches, Apple will always charge whatever is lower: the price it was pre-ordered at or the cost when it becomes available.
Although the App Store has largely remained the same since its launch in July 2008, Apple has made a number of changes over the past year and a half. In June 2016, the company finally introduced subscriptions and began allowing developers to inject paid App Store listings within search results. However, there's still no option to test apps before you buy them.