Travelled from Bangkok to Siem Reap on 22 Feb 2013 by direct bus.
Tickets sold from booth 22 at Mo Chit bus station. Get evisa first and take your passport if you go to buy a ticket in advance.
750 baht for the ticket. Bus left at 9.00. No stops or pick ups on the way. Water and a snack given out. Bus a/c with toilet. Brief stop at a bus stop in Aranyaprathet. Arrived at border at around 13.40. Despite assurances to the contrary we changed buses at the border but it was literally get off one and straight on the other. Probably due to driving on the left in Thailand and the right in Cambodia. Lunch snack served, pork and rice. Lots of people got off as they didn't have visas. They went to get visas at the border and make their own way onwards. Crew didn't speak much English but several Thai passengers did and were very helpful explaining what was going on. However, at the border an English speaking crew member joined us. All passenger Passport details were filled in on a form and then passports and visas plus 100 baht per person were taken off to immigration by the driver. We were given our passports back and got off the bus to clear Thai exit immigration. The Thai side of the Border was not that busy but we went through the group tours lane. Got back on the bus and drove past the casinos. Saw people who were on the bus earlier but didn't have visas in a queue to get through Cambodian immigration. Drove into Poipet and then pulled over just after the roundabout and the driver took the passports to Cambodian immigration. We sat and waited on the bus and then passports arrived back. Departed at 15.30. Arrived in Siem Reap at 17.30 and took a $4 tuk tuk to the hotel. No hassle with the driver, accepted we had booked a hotel and didn't need him the following day.
Summary - Prolonged but comfortable journey and no hassle. Would definitely do it this way again, especially after seeing how people are targeted by beggars and touts as they cross into Poipet.