Colones is the official currency of Costa Rica, the exchange rate does not change a lot, because it is quite stable moving along at around the same rate. For more than a year it has been around 540 colones for each 1 USD. It is not hard to find a place to exchange USD or even Euros with a fair rate, but there are a couple of recommendations to keep in mind.
Almost anyone arriving to a foreign country wants to have some of the local currency. That is a good idea in Costa Rica as well, but it is good to know that almost anyone around will accept USD as a form of payment, this even includes buses, taxis, restaurants, hotels and supermarkets, they may not have USD currency to give change back but they will use local colones.
I normally do not see a reason to have to exchange foreign currency into colones at the arrival at any of the two main airports. There is certainly a professional and friendly exchange service at the arrival of the international terminals, but their rate is quite bad and they charge a commission. One will lose a lot of money in that kind of transaction at the international airport. If even knowing that one still decides that it is important to exchange at the arrival just don’t exchange lot of money. Just recently an ATM exchanging machine was added in the airport next to the exchanging office in the arrival lounge of the San Jose international Airport. It seems that this ATM machine is able to exchange from USD to Colones with very good rates.
The best thing to do in Costa Rica is to exchange foreign currency at the local banks, they have the best rate and they do not charge any commission. Banco Nacional and Banco de Costa Rica are two of the local banks with branches everywhere. They normally open around 9am and close around 3pm. Another good option is to use ATMs, the rate there is good, one gets charged a little fee (normally five USD) but it is better than paying a commission in another place. Just be sure to use the official ATMs, preferably inside a bank. When using the ATM it is better to use a debit card because they charge a smaller fee, some people prefer to use a credit card because it seems to be safer.
If one is paying with USD around in Costa Rica, it is a good idea to always keep handy the official exchange rate, everyday this rate is posted in the main national newspapers, since the currency of Costa Rica is always large denominations with lots of zeros, it is as well good to keep a calculator and always do the mathematical operations by yourself. Costa Rica is a safe place, but like in any other destination around the world, be prudent and do not carry around or show a lot of money in public places.