I was there about ten years ago, and was amazed by the hordes of tour bus groups everywhere. One very useful thing to know is that the tour groups take a lunch/siesta break for several hours between about 11-12 to 2-3 (I'm only approximately remembering the times, but it's something like that), so the number of people at the temples thins out by about a factor of ten during those hours, which makes photography much easier. Of course, if you're on a bus tour, that advice doesn't help - in that case, just expect that you won't be able to avoid getting tourists in most of your photos.
My favorite of the temples, especially for atmospheric photography, was the Bayon (the one with the giant faces on the towers).
Be prepared for hot, humid weather. Being cold & flu season then, do what you can to avoid getting sick, and bring remedies so you can keep going if you get slightly sick - I managed to have a constant string of five colds during my two weeks in Thailand and Cambodia in late December. It seemed like an awful lot of people were sneezing and coughing everywhere we went...
Lisa