The swoopy styling looks good and the interior seems very nice, but the lack of rear headroom might be a deal-breaker for some as well.
Either the rear headroom numbers are misleading or reviews about the Sonata are misrepresenting facts.
Here are the rear headroom numbers for many sedans, in order of highest to lowest:
Passat 38.6"
Old Sonata 38.2"
New Sonata 37.8"
Impala 37.8"
Camry 37.8"
Malibu 37.2"
Mazda6 37.1"
Galant 37.0"
Accord 36.8"
TL 36.7"
3-Series 36.1"
Altima 35.6"
So... by only looking at the numbers, the Sonata went from having the most rear headroom to being tied for the most rear headroom with the Impala and Camry.
Like I said, unless these numbers are really misrepresentative, I don't understand why people keep saying the new Sonata lacks rear headroom. If it does, then the same is true for every other sedan out there.
EDIT: Just looked it up and the Passat tops them all with 38.6" of rear headroom. Still doesn't change the fact that the new Sonata is at the very least very competitive with the Camry and Accord, it's main competitors.