After all the recommendations, I am currently on the 8:30 Mekong express from Siem reap to Siem reap to Phnom Penh.
The cost was 12 dollars which is twice the price of the competing VIP busses.
Over other vip buses you get breakfast and a bottle of cold water and a significant amount of leg room and wifi.
Unfortunately the negatives about the bus far out way any extra benefits.
The first issue is the ac, they put it on for an hour and then you have to do the next 5 and a half hours without any. I asked them and they said it was used up. I'm pretty sure the just put a block of ice in the fan unit at the back as it has a garden hose coming out the back to drain the water.
The breakfast was a whole packet of rich tea biscuits, now I'm a fan of a biscuit with my tea, but a whole pack with a bottle of water to wash it down, well its not really breakfast is it.
The third issue is the bus, it is old and crappy, they have done it up with some dangling fabric, but this bus is at least 40 years old.
The last issue one is the half an hour stop, it was near a rubbish restaurant the "recommended", that was over priced .
They did not lock the bus of the luggage compartment, so when I headed back to the bus, I saw a guy local climbing out the luggage compartment when, when he saw me he ran behind the market stalls.
Others may have had better experiences, but at this price I cannot recommend doing this trip with this bus company.