The Best Way To Phnom Penh From Siem Reap Is....

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.

The highway from B'bang to PP also has no divider but the road is in considerably better shape. I am going to PP in an hour but will have to brave the SR - PP highway as going via B'bang adds too much time. But if time is no factor, this is certainly the way to go.

Battambang to Phnom Penh by minivan is about 5hrs. The road is good until you get close to Phnom Penh. The drivers are going about 120 km/hr for a fair bit of it so maybe best to keep your eyes closed.
Did the boat from Battambang to Siem Reap last week. 7.5 hrs but quite a good trip (much better than when I last did it 3 years ago). I think it takes longer if you start from the Siem Reap end because of the current.