I know this is a very late reply, but I thought it might be useful to someone else. I just did the drive trip through Costa Rica in the Fall of last year - we stayed in Jaco Beach and drove up and down the country. While it is not for everyone, I don't think I would have wanted to be without a car in Costa Rica. (FYI- There were reports of a few thieves targeting car rental customers in San Jose by throwing down tack strips and pretending to help you change a tire while robbing you blind, but we didn't not encounter any.)
I have many suggestions for getting around and highly recommend buying a very detailed road map prior to leaving - the roads are not well marked. Yes, everyone here is also speaking the truth when they say there are pot holes the size of your car, and the little 4-wheel drives you rent sometimes have trouble getting up the hills. The drive to Arenal is winding but do-able and fairly smooth by comparison to others. Monteverde is a little more difficult. Everything on the map looks very close, but remember that you are going to be driving on narrow roads and will often be behind large trucks you cannot pass going 15-25 MPH. Budget HOURS of time to go 50-70 miles. The main highway is painfully slow because of the same trucks.
If you would like any more information, feel free to send me a private message. I really love Costa Rica and I bet you'll have a great time.