If you're going to Costa Rica I would definitely recommend bringing some really good maps, or even better, a GPS. I haven't seen one road sign with the name of a road the whole time we've been in Costa Rica, and the people here generally don't know the names of the roads either. That makes it really hard to look up directions, which usually tell you the name of the road you need to turn on.
The good thing is, if you can get headed in the right direction, most of the highways have signs telling you which direction to upcoming towns. Those signs are also very accurate, and it's best to follow them even if you don't think they quite match up with the map, or if the GPS tells you to go a different way. Sometimes the GPS or sites like google maps show that roads don't go through and try to tell you to take ridiculously long ways between towns, but if you just follow the signs you should be fine. In some cases we did find areas where the roads didn't go through and it was necessary to take a really long way around, but the signs still told you which was to go and got you to the right place.
As far as driving around town, it's usually pretty confusing if you're trying to go by street names. It's much better if you know landmarks you can go by - for example our guide book listed one tour guide office as being across the street from the town church. If you can, I think it's usually easier to just walk around town because it's easier to look for landmarks and read signs on buildings. Plus if you walk you also don't have to worry about bad traffic (like in San Jose & Liberia) or unmarked, one-way roads (which were quite numerous in Liberia).
We also heard before heading to Costa Rica that most of the roads aren't in great condition and it's better to have some sort of SUV than a sedan. I don't think the roads are quite as bad as people made them out to be, but it also really depends on where you want to go. The highways and roads in big cities like San Jose and Liberia were pretty smooth and well paved for the most part, although in certain areas of the cities or in smaller cities like Coco there are a lot of very large potholes. Also in the beach towns there were areas of the cities that had unpaved roads, which weren't terrible, but did get some bad ruts and puddles in the heavy rain. Other parts of Costa Rica, like the road to Monteverde or the road towards Playa Pan de Azúcar (which we didn't make it to), are fairly rough and an SUV is pretty much necessary unless you really want to bang up your car. The roads aren't too rough, but are very rocky which is why you need something with more clearance than a sedan. We didn't need four wheel drive on the way to Monteverde, but the roads get really bad in the rain and we did use it to get out of the mud on the road that goes towards Playa Pan de Azúcar. In general small SUVs seem to be the most popular cars around, but there are still plenty of people in the cities with little sedans. And motorcycles, especially dual sports, seem to be much more popular than cars in general.