I used the Giant Ibis busservice from Siem Rep to PP two years ago. Made the reservation and bought the ticket from the hotel I was staying at. Was picked up by minibus from the hotel and taken to the main pick up point and the bus to Phnom Penh. And yes , we did make a half hour rest stop along the way. Fine with me ,to get a chance to stretch your legs and get some fresh air.
All in all , I had only a good experience with the Giant Ibis: Modern busses and everything just worked from pick up at the hotel to arrival at PP.