For the 10 years I have lived in Thailand, taking a bus from Bangkok to Phnom Penh or Siem Reap, Cambodia was an incredible pain. It meant taking a bus from Bangkok to the Cambodian border, then at least one or two bus changes before you ever got to Phnom Penh or Siem Reap.
Now, thanks to a new bus service that will take you direct from Bangkok to Phnom Penh or Siem Reap, it just got a lot easier.
If you are looking for a direct bus from Bangkok to Phnom Penh or Siem Reap, all you need to do is head to the Mo Chit bus station in northern Bangkok early in the morning (take the BTS skytrain to Mo Chit station and then a quick motorbike taxi).
The bus from Bangkok to Phnom Penh leaves at 8:15am and will cost you 900 baht. The trip takes around 11 hours.
As for the direct bus from Bangkok to Siem Reap, that leaves 45 minutes later at 9am with a cost of 750 baht.
Return service from Cambodia to Bangkok also runs once daily — at 7am from Phnom Penh to Bangkok, and at 8am from Siem Reap, arriving in Bangkok in the late afternoon-early evening.
All I can say is, with this new bus service, it might just be time for me to take a long weekend trip to Phnom Penh or Siem Reap. It is a lot cheaper traveling on the bus than flying and, by the time you have checked in at airports and waited for your flight, taking the bus does not even take that much longer.
Buses are also air-conditioned, and stop every few hours for quick bathroom breaks, coffee breaks and to be able to buy snacks or meals.
All in all, a lot more preferable than by plane and no hassles with going through security or with the copious amounts of x-rays Thai and Cambodia airports often now demand.