I think DHacker, while disagreeing with me, implicitly confirms my point when he notes that the Ticos pass regardless of oncoming traffic. Given that, how safe are you no matter how carefully YOU are driving?
I should say that I was willing to, and in fact did, drive when I was outside the city, but I used a motorcycle for superior evasive capabilities when faced with oncoming Ticos in my lane. I did go on somewhat short notice but the best I found for a DECENT SIZED four wheel drive was $115 a day with all the taxes and including the collision damage insurance. But, I'm sure deals can be had and I only wanted it for I think three days.
Having said all that, if you drive in San Jose I just think you are nuts because taxis are cheap. A popular bumper sticker down there is "Save A Life: Teach a Tico To Drive." Plus ATVs, including 3-wheeled ones which are totally banned in the US for safety reasons, are street legal and very common in the central highlands, and if you are not used to that you could smush one of those low down little buggers real easy.
Just my opinion and I should note that I tend to try to find ways not to drive if I don't have to in pretty much all foreign countries. But do note that I was in a serious accident in a foreign country so I know first hand how incredibly much hassle it is. YEMV.
JD