I stayed at Smiley's for almost a week while I was in Siem Reap. You can't beat it for $5 USD/ night ($15, I think, for A/C and hot water). It's family run and the staff is super nice. They have two computers with free internet (a bit slow), good food at their restaurant, and it feels more like you're in an old, high end, hotel rather than a cheap guesthouse. Its also only a 10 minute walk or $1 tuk-tuk ride to the night market and Pub Street.

I've stayed in budget hotels in Thailand, China, and Cambodia, and this is the best budget hotel that I've stayed in. This was a school group, and we had single, double, triple, and quad rooms. My single room had a double bed (excellent mattress: like memory foam, that was an amazing surprise), air conditioning, older 13" TV (worked fine) with local cable TV including CNN and about 35 other channels. The room was very clean: shiny, ceramic tile floor. All our rooms (group rate) were very reasonable; under $20.

Bathroom has on-demand heater: took a bit to figure out to press the button which turns on the shower and then the heater comes on for hot water. Like many Asian bathrooms, the whole bathroom gets wet, so wear the (provided) slippers.

Location is about 12 minutes walk from the Night market, or 15 minutes from the old town market (or take a Tuk Tuk for a couple dollars). Seem quite secure, and the (family-owned) staff was very friendly and helpful. Restaurant was very good: $2.50. Didn't eat other meals there.

We stayed in the Smiley Guesthouse for 7 nights and only that long because one of us got sick. First of all, the rooms are really nice, big and have a comfortable feel. We saw two rooms because we had to move. In our first one, the tv didn't work any more because of the construction (every morning from 10-12, then long tv break, no working in the afternoon...) they had going on in another room. I guess they didn't realize that it was because of that. We told them, but they only offered us a new room the next day. So we had to move after 2-3 days. Pack all our stuff, move to another room. No sorry, no nothing. Just a 1$ more expensive room ("Room has fridge, so you have to pay more"). We lived with that because it was still only 12$ for a/c, tv, fridge but cold water. Hot water would be 15$, but you don't really need it in Siem Reap in March. The cold water was acutally always very warm - WHEN there was water pressure. One day we didn't have water anymore, so they flooded the whole bathroom to clean the pipes with brown, dirty water. After the water turned clear again, the guy just left - with the bathroom still totally dirty from the rust in the pipes. We asked him if he could clean it, and he was really surprised that we wanted him to clean up the mess. He did reluctantly, and left us with a half-dirty bathroom. Ok, lived with that and turned over the key the next day to have the total room cleaned (now we stayed here for about 6 days). We hung out the sign "Please clean room" and left for the temples. Came back: Room of course wasn't clean. We asked for new towels then, and the girl at the counter chatting on facebook said somebody will bring us new ones. After 30 minutes we asked again because we really wanted to get a shower, and she said, that the cleaning lady isn't there right now. So we asked if there is any way we can get a clean towel, and all of a sudden she got up and gave us new ones... She seemed really annoyed to have been disturbed. And talking about disturbed: When ordering breakfast or dinner in the restaurant, we could be lucky if we were heard, because the whole crew sits in front of the tv all day. They don't even start right now with preparing your order, when they want to finish something on tv. All the time, we had the feeling that we were disturbing them in their every-day-life by being guests in their guesthouse. Food was ok, but it was always a hassle to get it. Once, one girl even talked on the phone while taking our order and didn't listen...
The wifi didn't really work as it was supposed to.
I know, we are still in Cambodia and we have to deal with a couple of things differenty than in Europe. But we travel for 4 month now in South East Asia and this was the worst service ever. We can live with broken things in our room, with food not as good and bad tv reception. No worries - but please: At least even apologize and be friendly! ThatÄs all!

There was one highlight in the guesthouse: Our driver to the temples. He was VERY friendly and always observant and helpful. The best service in the guesthouse. If he hadn't been there, we would have moved a couple of days earlier. He even sometimes brought us the menu to order food (normally you had to get it yourself).

So, if you want to stay there, you should know, that the rooms are nice, the food is ok, but the service is the worst.

We had a lovely room for about $10; unlike many budget rooms in SE Asia it looked as though a bit of thought had gone into the decor, rather than just being furnished with cast-offs from someone's grandma. The restaurant was in a lovely partly open-air garden setting with big wicker chairs which were perfect for relaxing in the shade after a hard day's sightseeing.

Some of the staff weren't that helpful but it didn't really detract from our visit. We hired push-bikes for local trips for $1.50 a day and they were fine.

We discovered when we went out in the evening that the guest house is next door to a brothel, but nobody bothered us.