Typing this from Costa Rica
We are in Arenal right now (The Springs) on day 6 of an 8 day vacation. Started at the Marriott Los Suenos (actually stayed at the SJO Courtyard the first night). I, too, researched and debated on the car rental vs. driver option beyond what would probably be considered normal. In the end we've gone with a driver, except one day we rented a Yaris from National at the Marriott. $80/day which I know is an insane amount, full coverage even though I had a letter from my cc company, no hold on the credit card. This was to get ourselves to Manuel Antonio. That drive wasn't bad but you have to be on your guard all the time.
We live in California and have certainly driven mountain roads, Hwy 1 Big Sur etc. It isn't the same here. The one time when the weather was bad earlier in the day happened to be the day the driver took us from Los Suenos to Arenal. I know you'll be coming from a different direction, so maybe that route isn't as bad. But 3.5 hours of winding mountain roads, lack of signs, pouring rain, fog/cloud cover, lightening--boy were we glad someone else was driving.
My original thought was rent a car in La Fortuna to get ourselves back to SJO. Nope, we're going with the driver. The car rental one way fee, high rates, driving challenges . . . don't want to be that stressed on vacation.
One thing everyone tells you (including professional drivers), do not drive at night. Also, they guy who took us on the mangrove boat tour cautioned to be very careful driving back in the rain in the Yaris--if you hit a pot hole the car can hydroplane. I would never get a subcompact. Each time we venture out we see accidents. The Costa Ricans have no qualms about letting their kids sit on the side of the roads/hwys while cars speed by. There are speed bumps, pot holes, passing on solid lines ("major" roads are 2 lanes), policia, motorcycles, bicyclists, and towns every so many miles.
Sorry maybe I have done too much research and have too much fear; but what I read before coming here was not an exaggeration. The weather has been much less rainy than I anticipated. It starts about 3 p.m.--yesterday none, one day after dark--so weather is just fine. Very uncrowded. The Springs is gorgeous, I think the most beautiful place I've ever been (only staying here because the rate was 55% off). You are captive, however, because the 2 mile road in is unpaved rocks (way bigger than gravel).
Ah, I hear the toucan in the tree outside our room--off to try & get a picture of him.