[The Double Down's] 540 calories contain 145 milligrams of cholesterol (more than twice that of the Big Mac and about half of the USDA's daily allowance) -- along with 1,380 milligrams of sodium (the USDA recommends no more than 2,400 per day) and 32 grams of fat (65 will keep you slim, says the government). So, for getting only about one-quarter of the calories that you need in a day, you're exhausting about half your budget of "bad stuff".
We can, of course, be a bit more exacting about this. I've created an index based on the amount of fat, sodium and cholesterol that the Double Down and a variety of comparable sandwiches contain as a portion of the USDA daily allowance. (In the fat category, saturated fats are counted double and trans-fats are counted triple.) The index is scaled such that the Original Recipe version of the sandwich receives a score of 1.00, a measure of gluttony that will hereafter be known as The Double Down (DD).**
By this measure, the Double Down is indeed quite unhealthy, but some other sandwiches are just as bad.
KFC Double Down Unhealthiest Sandwich Ever? Nate Silver Breaks Down The Numbers