CAMBODIA
We visited Cambodia in December 2014 and found it a fascinating country with incredibly friendly people. There is plenty to see in Siem Reap and Phnom Penh and a good standard of hotels and restaurants.
Cambodia's history in relatively recent times is tragic, and there are opportunities to learn more about this history, for example by visiting The Killing Fields and S21 Prison in Phnom Penh. We read a few books about life under the Khmer Rouge before our trip and 
were glad that we had learned more about the horrors of Cambodia before going there. One particularly brilliant book is 'Survival in the Killing Fields' which was written by Haing Ngor, best known for his academy award-winning role as Dith Pran in "The Killing Fields" movie, but whose story of survival is harrowing but quite incredible.
We travelled to Siem Reap from Koh Chang in Thailand, and the journey was an adventure to say the least! A mini bus picked us up at our hotel on Koh Chang first thing and drove us to the Thai Cambodia border. It's a relatively comfortable journey and takes about 4 hours. The bus company will probably take you to a restaurant where they try to sell cambodia visas to people who don't have them. It's best to arrange an E-visa online before you go, as this saves the danger of being scammed. Click here to arrange your E-visa - currently costs $30 + $7 processing fee. The staff from the bus company tried to get us to change Thai baht into Cambodian riels before exiting Thailand, but don't do this, as the only currency you will need in Cambodia is US dollars, and the rate you get here is very poor.
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Depending on the time of year you visit be prepared to spend a long time at the border. It took us about 30 minutes to exit Thailand and then you enter a very strange area which is No Man's Land (neither Thailand nor Cambodia). The queue to Cambodian Immigration was pretty long and we resisted paying extra to the staff from the bus company for fast track through immigration because it sounded too much like a scam. However, when we were still waiting after 2 hours we started to regret not paying the extra! When we finally got through immigration there was a bus waiting to take us to the bus station, and on that bus were some of the people who'd been on our mini bus and had paid extra for fast track! The journey to the bus station took longer than we'd expected but at least our taxi that we'd booked in Thailand as part of the journey was waiting for us. We landed up sharing the taxi with a guy who'd planned originally to take the bus to Siem Reap from the bus station but had been waiting for more than 3 hours and wasn't prepared to hang around any longer! The journey to Siem Reap took a very long 2 hours - it was quite terrifying! It reminded us of journeys in India where you're over-taking and a car is coming towards you or someone is overtaking coming in the opposite direction and you can't think how they're going to get back in their own lane on time! Anyway, we made it, and were so happy to eventually arrive at our hotel!
We flew to Phnom Penmh from Siem Reap using Cambodian Angkor Air. It's a short flight of less than an hour, although you can go by bus which will take about 6 hours. We decided to fly because we didn't have a lot of time. We got a taxi from the airport to our hotel which cost $12 each way* and took about an hour when we arrived and the traffic was horrendous but the return journey just took about 1/2 an hour.
Useful forums on Cambodia
Trip Advisor Cambodia Forum
Fodor's Cambodia Travel Forum