I rented a car and loved it. I did drive over to the left coast from San Jose and was only extorted by the police once. Note: please, carry your passport at all times in the car. They tell you to just take the ticket. But I didn't want to piss the guy off. Call it a rookie mistake, but I just wanted to move on.
For the most part though, I really enjoyed it. It gave me the freedom to explore. Time stands still a bit on the beach and with a car, you can drive towards Quepos or more inland or wherever if you so feel. You don't have to wait for a driver or worry about a ride home. The roads are fine as long as you stay on the beaten path. I agree with the cons, but for me, driving with my ipod plugged in across some beautiful country was a great way for me to unwind. And I found it to be relatively inexpensive. But I guess that depends on how much you plan on traveling. I did get the full insurance where there was not even a deductible. I think it cost another $12/day or so. Basically, I prepaid my gringo extortion.
Oh, and if you have an SUV, you will have the largest car on the road. But what better way to enjoy the scenic view of CR than a little rh from your tica chica. Just stay on the road.
Oh, and you might think gas is cheaper until you realize that it is in liters. Again, another example of why I am not the smartest guy alive.