Although foodwise, Siem Reap is, as others have reported, a bit of a wasteland, we did manage to have a few good meals. I realize that coming from Vietnam, we were somewhat spoiled, though. The Khmer food we ate in Siem Reap was less flavourful on the whole and didn't use the abundance of beautiful fresh herbs and greens we'd enjoyed in Vietnam. But still, it was ok and the charming people compensated a little.
Our first night we ate at Old House which was actually the least interesting meal we had in Siem Reap. We had the $6 set menu, so it was inexpensive, but the food was flat and uninteresting. Location was ok, just off the Pub Street so not such a zoo.
On the second night we had reservations at Cuisine Wat Damnak and it was pretty wonderful. The French chef has taken local Cambodian ingredients and showcases them in a tasting menu that changes weekly (I think). The dishes are not actually Cambodian but incorporate the flavours in a really unique way. We thoroughly enjoyed our dinner which was served in a beautiful leafy garden. Ridiculously great value - $24 for 5 courses or $28 for 6. Highly recommended but you MUST book ahead.
Had a couple of meals at Khmer kitchen - 2 different locations, one in town and the other one out by the temples. The one near the temples was better, we thought, but both were cheap and edible.
And our last dinner was at a really lovely place called Haven, which is a training restaurant for disadvantaged youth. I am usually suspicious about places like this, which seem to be very common in both Vietnwm and Cambodia, figuring they are overpriced and bland. But this one wasn't either of those things. The food was nice, fresh, beautifully presented and very inexpensive. Our servers were charming and attentive and the garden dining area was comfortable and breezy. I would recommended this place also - but it is worthwhile to book ahead as they're really busy.
On to Phnom Penh and beyond!