Crossing the border from Vietnam into Cambodia was painless and easy. I took a bus from HCMC to Phnom Penh - the bus attendant takes your passport and visa fee and then basically does everything for you. I did have to get off the bus and go through the line, but we were all bumped to the front - got my photo taken and fingerprinted and I was back on the bus in less than 10 minutes.
You're hit with a blast of poverty upon crossing the border - I encountered a few beggars and saw some desperately thin, dirty people. Then I saw a truck packed full of live ducks and chickens - they ended up on the ferry with us, and so I saw the birds up close - it was a sad sight for me, hard to comprehend. The chickens and ducks were obviously terrified, some were dying, drooping their heads, many ducks were panting and looking up at the sky - so close and yet so far! Their feet were tied and anchored to the rails on the truck. It was a horrible sight, but somewhat natural for the local people and maybe not even intentionally cruel (though I did see one of the guys sitting on the truck reach over and ring the neck of a chicken when it pecked at him) - and I was thinking about how ducks, chickens and other animals bred for consumption are mistreated in my own country only it's not publicly visible, which is just as bad.
I arrived in Phnom Penh, stayed at a backpacker hostel and wandered around the city in awe. Jaw-droppingly gorgeous architecture. It's pretty clean as well and I found most of the people smiling and friendly - there were vendor hassles and kids selling stuff and beggars - but for the most part it was all just interesting. Dollars rule here so you don't need to exchange for riel - everybody takes (and wants) your dollars. I discovered Cambodian cuisine is very much like Indian curries - wonderful! I was able to find vegetarian Amok, in fact, vegetarian fare was available everywhere. Monks were everywhere too, a beautiful sight!
I hired a tuk-tuk driver for the day to take me to the Killing Fields and S-21 - it was a scenic trip through the city and the driver waited for me as I took as long as I wanted in each place. Both were unforgettable experiences, especially the Killing Fields for the way it was done. When you buy a ticket (cheap) you're given a headset - each spot is numbered so you stand there silently listening to the narrative and just feeling the place. The narrator was a gentle male voice who described what happened - in some spots you listen to survivor tales in their own words (translated), and there's an incredibly beautiful classical song that you can listen to as you walk the path around the little lake. It was an amazingly moving experience, and I felt so fortunate to actually be there, experiencing it. It was a gorgeous day - breezy and blue, the air was filled with birdsong - this terrible place is now a sanctuary of peace. Since everybody is listening via headphones they are basically silent and respectful. I don't think anyone could walk through there without crying.
S-21 was stark and still frightening - nothing was changed, only the bodies were removed. I realized that all those awful spots on the walls and floor were old blood spatter stains. I spent a long time looking into the eyes of the victims - just row after row of pictures. I'd read an account of one of the survivors there (there were only between 7 or 11) and he described what happened in these rooms and cells so vividly, so I had a real sense of what I was looking at as I wandered through. One thing I've loved about Cambodia so far is the lack of a structured "tour" - you move about on your own without much in the way of restriction, and nobody is telling you what to think or take a picture of - two things about organized tours that I abhor.