We rented one and it wasn't as expensive as we thought it would be. You do have to pay for a mandatory insurance though. One of the insurance options should be covered by your credit card. We used Vamos and they were great to work with. We were very happy to have the car as we stayed in places that were not within the town center. The Arenal area is quite large and we were able to easily get to all the sites. Vamos provided us with a cell phone so we could use Waze to navigate.
No issues with driving and we were so happy to have a car and would rent one again if we go back.
We used Airbnb-there were so many options. We landed in Liberia, got our car and drove to Playa Hermosa, then to La Fortuna and explored the Arenal area, then to Monteverde, Samara and flew out of Liberia. Easy driving, the scenery is beautiful and we loved being able to get out and take pictures at scenic points. Keep in mind the distances don't seem too long, but because the roads are very windy with sharp turns, you don't go very fast. Don't drive at night.