We took the VIP bus a few years ago and quite enjoyed it. The main drawback if you have luggage however is that the bus stations in both CM and CR are a fair distance out of the center of town.

We often rent cars and don't mind driving in Thailand at all but don't really need a car in either of the two cities and don't want to pay a one-way charge (not to mention the hassle we've had with car rental agencies in the past).

For that reason we now just hire a car and driver.