I took a bus from Siem Reap to Phnom Penh yesterday (28th July) and the road had considerable construction. It took about 7.5 hours. In hindsight I probably would have been better off going to Battambang by boat and staying for a of couple days, then going on to Phnom Penh. The boat is running again and it's an interesting journey past floating villages, etc.
Maybe someone who has more experience in Cambodia can verify this, but it seems the highway from Battambang to Phnom Penh is much better (i.e. safer, faster). The road from Siem Reap to Phnom Penh was not a divided highway, which made passing somewhat hair raising at times.
If you have the time, stopping off at Battambang might be a better option. Battambang has some nice countryside and is probably worth a couple days.
