I visited the Ayutthaya site in April 2015 as a day trip from Bangkok. I took a private taxi to get there and take me around to be able to see as much as possible. The UNESCO sites are on the island only; and not even all on that island are UNESCO. I saw the most important, but also two great sights outside the island.
The heat was extreme, but the temples were great. They have done a lot of restauration in the last years and everything is wel kept. Of course, it's ruins, but they are great. A lot was also build of wood which of cours eis gone, and also most relics you cannot see and the paintings inside the temples you cannot access too.
A great place for a day trip from Bangkok.
please see my website for more photo's and the route I took on a GPS map.