I had not used one for a while, but your question prompted me to do so. One of my favourites is the Readability Score [com], and I tried four of my articles there now.
I seem to keep to a certain level and I am not sure whether this is good/bad/ or just plain 'forgettable'. The Flesch-Kincaid Reading is usually bet 74 and 79 and the avg I get for readability score is in the Grade 6 . something.
The way I see it is that it all depends WHO you are writing for. Would it make any difference to you if you are writing for a young teen, or for a schoolteacher or for an academic? I tend to think that even if they can each understand your article, there is still the question of "appeal".