I was there for a few days in March and it was extremely hot and humid. But I brought quite a bit with me - 2 bodies, a tripod, and usually had 4 lenses with me at all times. I found the tripod key in several instances - twilight and sunset where shadows were pronounced. And that made an graduated ND filter very important. In hindsight, if there are a lot of crowds cluttering up your shot, a very dark ND filter combined w/ a long shutter speed may be enough to make the crowds disappear. The highlight of my trip was sunrise at Bayon... truly amazing.
The mosquitoes are voracious - bring lots of bug spray and sun lotion if you burn easy. There were a couple times when I was too absorbed by shooting and I wasn't drinking enough fluids... it's hot, you will sweat a lot so drink a lot! Heat stroke sucks and will waste a lot of valuable time. Plus it doesn't feel too good either
Hope your trip goes well!