Two days ago I travelled from Siem Reap to Phnom Penh with my father and sister using Golden Bayon Express, at the suggestion of our guesthouse that it is a safe and reliable option. The vehicle was a late model Toyota Hiace 12 seater van.
They drove at extremely high speeds - an average of 100 km/h and frequently at 120+ km/h (I could see the speedo), at least in the first section to Kompong Thom, overtaking and passing every other vehicle on the road. It was by far the most dangerous and aggressive driving I have experienced anywhere in the world and was far too fast for the road conditions and level of traffic.
I am quite tall and was very cramped for leg room, so at the lunch stop at Kompong Thom I jumped ship and was lucky to score a seat on a normal "large bus" for the remainder of the journey. My father and sister stayed in the van and had the "daylights shaken out of them" on the rough second half of the journey after Kompong Thom.
Other Khmer passengers explained that the drivers must abide by company policy to complete the journey in a maximum of 6 hours. They have a 30 minute break in Phnom Penh and then pickup new passengers and return to Siem Reap the same day. The worrying part of this scenario is that the latter part of the return trip will be in the dark. To quote the driver himself "he must drive fast on the first half of the trip to meet the time schedule, because of the rougher, slower Kompong Thom to Phnom Penh section".
In three previous visits to Cambodia I have had great success using normal "large buses" which are slower but are safe and comfortable. If you value your safety and comfort, I would advise avoiding these Golden Bayon Express vans.
