My wife and I spent 2 weeks in Costa Rica for the first time in December of 2012. We rented a car (despite all the warnings) and dove to Tamarindo, Montezuma, Arenal, and spent 3 days backpacking in the Corcovado National Park. Montezuma was amazing, but I have heard tourism is taking a toll there (so visit now, before it's gone!) The Corcovado (most bio-diverse place on the planet) was insanely awesome! Highly recommended.
We tend to travel on a light budget, and like to get off the beaten path. We ran into a few problems driving, but I'd do it again. Except for one night when we got caught in the fog after dark. I'd also pack lighter for the Corcovado.