Travelling from Vietnam to Cambodia was probably the easiest border crossing I had so far. Below is my travel experience from Ho Chi Minh City, Vietnam to Siem Reap, Cambodia . Hope this would help .
There are a number of bus companies travelling HMC to Siem Reap route which takes around 10 hours (500 km) between two cities. I booked my ticket with Sinh Tourist Inc, which is conveniently located near Bui Vien St. where I stayed in HMC. Ticket was at 23 usd, leaving HMC at 6 AM. I booked this a day in Advance. There were other bus options available and some are cheaper by a dollar or two, but since I am travelling solo, I prefer to take the earliest departure time available to get me to Siem Reap before dark.
Upon crossing the border, one can immediately see the distinctiveness between the two nations. Vietnam has stretched its commercial development up to the border while the surrounding immediately changed to rural scenes and flat lands the moment you reach Cambodia. The architecture also changes, from the Chinese-inspired buildings in Vietnam to Cambodia’s shrines and residences more similar to Thailand.
We left Ho Chi Minh at around 6:30 am and Reached Vietnam Border at Moc Bai in 2 hours , the bus staff took our passport and assisted us in the Vietnamese immigration, then we hopped on the bus again to the Cambodian immigration which was just 200 meters away.
The Cambodian border was too relaxed, our baggages were not even checked . We were also asked to pay 2 Dollars for processing and stamping fee. Some said this was tout by that immigration office but I did not bother to argue about it.
We got our passport back and left the immigration building ,crossed the Mekong River for about 15 minutes by ferry and travelled for 3 hours to Cambodia’s Capital , Phnom Penh . We arrived at Sinh Tourist Office in Phnom Penh at around 12 nn, had lunch and a quick look around at the nearby market , while waiting for our bus transfer to Siem Reap at 2 pm..
Central Cambodia is generally flat , rice fields , with some vast patches of swampy areas. Road itself was OK. Some parts are a bit bumpy , but I dare not complain since I am used to travelling by bus here in the Philippines and some provincial roads are worse .
We arrived in Siem Reap at 7 pm , at the Sinh Tourist office opposite the Angkor National Museum . Siem Reap is relatively a city which is easy to navigate with the help of google map. Cambodians generally accept US Dollars, even for fares and in public market purchases.
I took a tuk-tuk for 1 USD to the Central Market where a lot of guest houses can be found. Streets were wide and well lit.
I did not make any prior hotel reservations since I was looking for decent affordable dorm types and wanted to check my options first before booking . But for convenience, pre booked hotels are ideal .

Angkor Thom Guest House near Wat Bo. , Siem Reap at 5 USD/night http://www.angkorthomguesthouse.com/