I'd also give Midnight in Paris four a half stars, incidentally.
But this isn't a review of Ernest Hemingway or Midnight in Paris, it's a review of an application called Hemingway, which you can find here. Hemingway the application is a free web app (though there's a purchasable desktop version that appears to do more or less the same thing) that describes itself as "like a spellchecker, but for style." Basically, you copy into it or write a section of text and Hemingway judges that text according to five parameters. Four of these get immediately identified with some neat colour coding: hard to read and very hard to read sentences are marked in yellow and red respectively, phrases with simpler alternatives are purple, adverbs are pale blue and incidents of passive voice are green. Lastly, Hemingway assigns a readability stat based on what I assume to be the US school grading system.
This is helpful, without a doubt. In fact, ever since I discovered Hemingway a couple of weeks ago I've been using it more or less constantly. There's something awfully brilliant about colour coding: paste in a paragraph and you'll see either a panic-inducing splatter of primary shades or a reassuringly plain background. It makes for an intuitive insight into what's working and what isn't; go mad with the adverbs, inadvertently phrase half your story in the passive voice, and Hemingway will let you know about it in no time at all.
Which is not to say you won't want to punch it. I spend approximately sixty percent of my time with Hemingway wanting to punch it, and I'm a fairly laid back sort; your personal mileage may vary. The thing is, as much as everything that it's pointing out is useful in theory, it's really not that bright. Unlike more sophisticated tools, it has no structural or contextual understanding of what you've written, and works off hard and fast rules and what appear to be fairly simple metrics.
Take adverbs, for example. Here's what Wikipedia has to say about adverbs:
Wow, Wikipedia, you make adverbs sound pretty awesome! But here's what Hemingway has to say about adverbs:An adverb is a word that modifies a verb, adjective, another adverb, determiner, noun phrase, clause, or sentence. Adverbs typically express manner, place, time, frequency, degree, level of certainty, etc., answering questions such as how?, in what way?, when?, where?, and to what extent?
ADVERBS ARE THE ANTICHRIST AND YOU SHOULD DELETE THEM, EVERY LAST ONE, BEFORE ALL GOODNESS IS SUCKED FROM THE WORLD AND SATAN RULETH FOREVER!!!!
4½ / 5
All of which is to say, you should totally check out Hemingway, it's a great tool, named after a great writer, who was impersonated in a great Woody Allen Film. (Perhaps the last great Woody Allen film?) But ... you should treat what it tells you with a pretty big pinch of salt. Frankly, all of its categories are about as dumb as those poor adverbs; its idea of what constitutes a hard to read sentence will have most authors wincing on a regular basis. On the other hand, if an entire paragraph turns red then you might want to consider reining things in a little.
This brings me a wider point that's a good one to wrap up on: though there are certainly a ton of great answers out there on the subject of writing, and no end of neat tools, none of them are ever entirely, one hundred percent right. A big part of being a successful writer, in my experience, is learning whatever you can from a particular source and then knowing when to disregard it. With that in mind, Hemingway is a pretty great editing tool; used with restraint, it definitely has the potential to make your writing life easier. Just don't altogether trust it, and certainly don't rely on it exclusively, that's all I'm saying.