Last January's trip was the first time we used Extra Magic Hours and we did it at both MK and Epcot. For MK, it wasn't really worth it. The park was gorgeous at night, but it was by far the most crowded we saw any park during our seven day stay. The lines were not shorter than during the day and the walkways felt claustrophobic. Epcot, on the other hand, worked out great. Epcot at night is so magical anyway and the crowds were noticeably thinner.
For this February's trip, I want to try to follow some advice I've read here and visit parks (especially MK) the morning after EMH.