If you are coming here for a trip, you might be tempted to buy colones before you get here. My advice is DON'T! The exchange rates are so poor abroad, and US dollars so widely accepted here, that there is absolutely no need to do this. You can pay for almost anything in dollars here. If you really feel the need to exchange your money, there's a bank or two at the airport, and you can do it there. It is easier to pay for things in colones, but there's no reason to make the exchange before you arrive in Costa Rica.
Buses are an exception. You have to pay with colones on public buses, as far as I am aware. And they will bitch and moan if you pass them a large bill (anything over 1,000 colones), so it's best to have a few thousand on you if you plan on taking buses. Cab drivers, especially airport cabs, will usually take dollars, though I'd play it safe there and ask before getting in. Again, cabbies will pitch a fit if they have to break a "large" bill (why the equivalent of $10 is considered large, I can't say, but that's just the way it is!), so have small bills on hand for paying cabs. If you're taking private transpo (like Interbus), you can most likely pay with dollars.
The other thing to know is that if you give someone a $10 bill to pay for something in dollars, you will get colones back. And you cannot pay for anything with coins; again, don't ask me why, it's just the way it is. Make sure you know the exchange rate if you plan on using dollars, or you may get ripped off. I have to admit I've been ripped off this way, and I even knew what the rate was. It wasn't much, but just the fact of the matter is enough to piss you off. When the rate is around 500 colones to the dollar, it's easy to calculate in your head. Or pay in colones and you don't have to worry. If you are paying with a credit card, just make sure the exchange is correct before they run your card. Having a transaction reversed or removed is a serious pain in the ass here and can take up more of your time than you may wish to give (ask me how I know).
I guess that's about it! Unless I've forgotten something, which is entirely