From a taste standpoint, the main complaints that people are having about the sandwich seem to be: 1) can't taste the "fillings" on the fried version, as the chicken overwhelms the rest of it 2) it's salty (no surprise as it's quite high in sodium, 3) it sits heavy in the gut. On the other hand, many admit to liking it.
As far as nutrition goes, for all the complaining, the Double Down has fewer calories than a Big Mac (fried version, 540 calories; grilled version 460 calories) and comparable sodium. The fat we don't worry about (for those new to this site, the evidence against fat is weak at best). I don't recommend fast food restaurants as great places to get healthy food, but if you're going to hit one, at least the Double Down won't send your blood sugar though the roof.
Update: Check out the comments section for an ingredients list. KFC's food does have a LOT of junky additives.
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