Ooh fancy. After a lot of deliberation where we weighed up the costs, time saved and comfort, we decided to take a taxi all the way from Sihanoukville to Phnom Penh's airport. This was one of the best decisions we have ever made!
Our cab driver picked us up at the time we requested and the taxi, which is just a sedan car, was clean, comfortable and air-conditioned. We hopped in and spent three and a half glorious hours driving from Sihanoukville to Phnom Penh international airport.
We stopped half way for a rest for both the car and our driver. We stood around discussing how riding in a private taxi is awesome and that we shouldn't get too attached to it as it isn't normally in our budget. Soon we were off again.
For $45 USD for a three and a half hour drive all the way from our hotel to the airport, it was money very well spent. We highly recommend you consider this option. The alternative is a tuk tuk to the bus station, waiting for the bus to leave, a long slow bus ride, dealing with the touts in Phnom Penh and getting another tuk tuk to the airport.
Yes, it will be cheaper but it will take far longer and be a lot more aggravation. Having a taxi pick us up at or door step then drop us at the airport was gloriously stress free. By the time you add the price of two tuk tuks and a bus the taxi isn't even all that expensive.