A "good" score, of course, depends on your target audience, your chosen style (part of branding), and what you mean to do. But there are some general rules of thumb. According to Future Now [2006] referring to various published studies, an "ideal writing standard" might be as follows:
No more than 4.25 characters per word.
No more than 5% passive voice.
No less than an 80% readability.
A Flesch-Kincaid Grade Level between 4 and 65 (sky's the limit, it seems...)
A Fog Index higher than 13 puts you in the danger zone.
While the Grade Level seems arbitrary depending on who your target audience is, it should be said that the average best-seller is around Grade 4 level, newspapers about 6, and business books 7-8. Lower than you might have thought.
Of course your audience can read at higher levels -- but you want to make your message easy to read. If your readability scores seem wildly out of line with your audience's abilities, Future Now offers a few strategies for making repairs: