There are several bus companies that operate between Siem Reap and Phnom Penh. The Mekong Express bus is the best of the lot and cost $10. The seat spacing is excellent and for the average person there is at least 6 inches of clearance from the knees to the seat in front. You are greeted by a lovely Khmer girl, shown to your seat and served pastries with a bottle of water. There is a toilet on board and you make one stop on the way for a break. They leave at 7:30, 8:30, and 12:30 and take around 51/2hrs over a good road, at least by Cambodia standards. Have your hotel help to make arrangements (the day before is normally sufficient) and the bus company will pick you up at your hotel. Be warned that the SR bus station is a disgrace and for many can be an overwhelming; unpleasant and chaotic.
As for comparison to buses in Thailand they they are not like the VIP ones but are on a par with the regular government a/c buses.
Another alternative is to hire a whole taxi for around $45-50 for the trip. This provides you with the option of stopping when you see something of interest.