Lonely Planet puts it this way:
The sacred city, the sacked city, Ayuthaya is complexly intertwined with Thai nationalism and religion. As the former royal capital, Ayuthaya earned the emerging Thai nation a place among the great empires of Southeast Asia, and placement on the map used by the international merchants sailing between India and China during the era of the trade winds. As a city that was destroyed by an invading army, Ayuthaya is eulogised like a slain hero.
But what is recounted in history and legend is not easily recognisable today. The ancient monuments retain little of their bygone majesty, with only a handful in recognisable forms. The modern city that grew among the rubble is busy and provincial, adding a distracting element of chaos to the meditative mood of crumbled kingdoms.
I spent some time biking around the ruins of Ayutthaya, and they were definitely beautiful. Lonely Planet captures the essence of the city pretty well in the above quote. Here are some photos so you can get a glimpse for yourself.