I have a house in Costa Rica, and drive when I'm there. Driving in San Jose can be quite interesting, but nothing that a sensible and intelligent driver can't handle.
As has been mentioned, the roads can be in poor shape outside of the cities, especially on rural (non highway) roads. Don't drive at night unless you're familiar with the road. Make sure you have your license, and passport with you, as the local police will expect a tip if you don't have them on you. (I've never paid more than $5)