Visit http://www.talesofasia.com to get great commentary on traveling between Bangkok and Siem Reap (and lots more great info about Cambodia). The first half an hour out of Siem Reap is not bad, the next 150 minutes varies between tarmac and dirt/sand. It wasn't that bad on our journey, but I could see where it would suck if there had been recent rain. About the worst you can expect in decent weather is a crazy driver - we were convinced that our driver would spin out or go over the berm when taking the detours around the construction at the bridge sites at 60 kilometers/hour. We made the journey from Earthwalkers in Siem Reap to our hotel in the middle of Bangkok in about seven and a half hours - including time spent at Poipet going through customs (maybe 15 minutes) and arranging for a taxi to Bangkok (1800 baht). I wish I had thought to do this earlier, but about an hour into the trip, I started snapping photos every five minutes. Click on the "MAP THIS" button in the upper right hand corner of this page and you'll see the photos on Google Earth and can get a good sense of the road conditions in each area. I didn't bother with photos on the Thailand side as it is all a very good paved highway. We were doing 120 km/h all the way into Bangkok.

:)