Above: Highway 6 in Kampong Thom Province. Parts of Highway 6 regularly wash away in the monsoon (around June and July), so until they actually get around to paving the lot of it, expect it to be rough. It's a lot of fun, but no Sunday ride.
It's really a total mixed bag of conditions. Between Poipet and Siem Reap there are extensive road works in progress (Feb.02), and it's in good condition, relatively speaking. After Siem Reap the road starts to fall apart, and by the time you get to K Thom Provence the degeneration is complete (see the pic above). From K Thom into Phnom Penh the road improves and it's a fast run into the capital through Skoun.
Note that in actual fact, Highway 6 starts at Sisophon. The road from Poipet to Sisophon is Highway 5, which then turns south to Battambang. However, don't let the maths confuse you. If you're heading out of Poipet on your way to Angkor Wat (Siem Reap), just keep heading due east. You can't miss it!