How I Use It
If you want students to have a little fun trying to write like Hemingway, this website could be an interactive way of cutting out the adverbs and simplifying the syntax and diction. That said, it can be very frustrating to advanced writers to have their sentences coded in colors being told their sentences are hard to read, when they are perfectly well crafted sentences! On top of that, Hemingway doesn't pass the test. The man oft used passive voice and students will quickly challenge the website once they realize that Hemingway's work includes "very hard to read" sentences. Heck, the one page I grabbed from "Old Man and the Sea" to analyze found an adverb!
The only place where I see this being valuable would be for journalism. Being succinct and using active voice are hallmarks. When it comes to mimicking an author's style there is more to it than syntax, length, and avoiding adverbs. Style is a complex thing and the website is not going to be able to replicate it.