First, virtually all American car rental companies are represented at the airport. Yes, there are a few that you haven't seen in the U.S., but the normal Budget, Avis, Hertz, National, and so forth are there, So, if you have had good experience with any of those companies, you might feel comfortable continuing to use them.
Second, there are smaller companies that we have had good experience with. One is Toyotarent, which as the name implies, primarily rents Toyotas. We recommend that you use a travel shopping web site like Kayak or Expedia to find the right combination of price and vehicle.
Third, we think it makes sense to rent medium sized SUVs for most uses. We like the Toyota Rav4, but other models would also work. Relatively good on gas, rugged for the bad roads, and more likely to be survivable if the yogurt hits the fan. Many, many roads in Costa Rica are dirt and gravel, which turns into mud in the rainy season, and therefore a 4WD vehicle is required to give you access to some neat places.
Comparing prices can be difficult because of insurance costs. There is the mandatory insurance (liability) and then the extra insurance (collision, comprehensive). We've never quite figured out the right answer, but some credit cards (AmEx) will cover one or more of these insurance costs if you charge the rental on your card. While all the prices seem cheap initially, we find that it ends up costing $50-$90 per day after insurance, taxes, etc.
Fourth, our place is about 150km away, so buying the notorious fuel option can be a good idea. Fuel is expensive, so you don't want to return the car with half a tank. On a recent trip, we were upgraded to a Toyota Land Cruiser Prado (large, but relatively simple SUV with a diesel engine. We opted for the pre-paid fuel and drove our entire five day trip on a single tank of fuel. There was 1/8 tank left when we returned to the airport. Your mileage may vary. We generally fly out early in the morning, so we leave the car at the airport (by prior arrangement). So, not having to get gas is a big plus.
Hey, it's pretty much like the U.S. but there are a few differences. We have seen few examples of aggressive driving, as in road rage or purposefully dangerous driving, but we do see a lot of drivers taking chances. This shows up as passing cars and trucks in low-visibility locations, or taking corners too fast and too close to the center line, and so forth. This boils down to the old Driver's Training admonition: Drive Defensively. Assume that the other driver is going to take his half out of the middle. Presume that there are people crossing in the dark up ahead. Don't rely on blinkers or tail lights or following the speed limit.
Three specific things that you should be aware of: many, many multilane roads narrow, and the sign will say "Ceda el Paso". You really don't have any rights at this point so don't meekly merge into faster traffic. Come to a stop if you have to. This calls for advance planning, and getting into the left lane sooner. Don't wait until you see the signs.
Some two-lane highways will switch to four lane at major intersections with on- and off-ramps. Trucks and slow vehicles will not pull into the slow lanes, and it's infuriating if you're used to "slower traffic merge right". In this case, however, it seems entirely OK to pass on the right and keep moving. Don't ever expect a slow vehicle to pull over to let you pass.
Second, some Tico drivers drive extremely slowly and will not, under any circumstances, pull over the let you by. You must pass them which, since they are driving 20 miles per hour, is not difficult. But your high beams, horn, and gestures won't do a damned thing. It's both sweet and infuriating. Don't fight it: pass when safe on the right or left.
Finally, the roads are reasonably good between the airport and our place, but at night they are very dark and lack lines on the road (except at Jaco which is a Christmas tree by comparison. You need to use your high beams to see any appreciable distance down the road. You're going to dim them like you do in Canada and the U.S., but this will happen every 15 seconds. It's more akin to driving in a cave than driving in the open air. You can do it, but you'll only find lit roadways in cities and at some intersections.