I spent the 2010 new year's weekend (actually only one and a half days) in Seim Reap visiting the Angkor Wat and the surrounding temples. The experience was fanatastic, thanks to the incredible organization of my guide Sowannna Yun (www.cambodianguide.com).
We managed to cover the Angkor Wat complex, the Bayon Temple, the Tha Prohm temple while I was there. I admit that this is not enough time to appreciate the full beauty of this magnificent area, but if you only have a weekend to spare, it would still be worth your while. Ideally, I would spend about 4 or 5 days in and around Siem Reap.
My local guide, Sowanna Yun, made my trip possible. He took me around in an airconditioned car (he also has access to motorbikes and tuk-tuks). His connections allowed us to drive into places where other cars (except police) were prohibited - this saved a lot of time and I got to see a lot of extra stuff. His English is very understandable; his knowledge of Cambodia history and the significance of the temple carvings/statues was amazing; and he knew exactly what pictures to take. I simply gave him my flight itinerary before I left to Cambodia, and Sowanna booked my hotel and arranged my whole weekend.
Unfortunately, I suffered from mild sunstroke on my first afternoon. Sowanna came with me to the hostipal and waited to make sure I was okay. If it wasn't for him, my trip really would have been ruined.