There are a few blog posts about the new direct bus from Bangkok to Siem Reap including …wordpress.com/tag/bus-bangkok-to-siem-reap/ and …wordpress.com/2013/… but not a lot of info going the other way.
To use this bus service from Siem Reap to Bangkok, you can buy tickets in advance from the departure point of the Nattakarn office a couple of shops north of the KFC on Sivatha Rd. Cost is $28. They indicate they can pick up from your hotel but my driver didn't show up so I took a tuk-tuk for a $1. Departure time is 8am. When I used it, Apr 2013, the bus was less than half full. It took 2 1/2 hrs to get to the border, where we had to exit and clear Cambodian customs on foot (head for the blue departures sign). We rejoined the bus which took us to the Thai customs side where we had to grab our luggage. We bypassed the outside queue and were led to the inside queue saving a couple of hours though it took about an hour to clear everybody on the bus since there were only 2 officials inside processing passports. Exiting the building, we had to get our luggage x-rayed and then met up with the bus on the other side. Had quick stop at the bus terminal to pick up more passengers and then served a light meal. Four hours later, we were at Northern Bus Terminal arriving at 4pm. From the terminal, follow signs to city buses which depart by the 7-11 beside the adjacent market. Take bus 3, 9, or 77 to BTS Skytrain (major stop and you should see the station overhead). Though transport from tour companies is much cheaper ($10-$12), you could be delayed for hours at the border and you could also switch to uncomfortable minivans.