The official currency is the colon, but American dollars are usually accepted as well.  You are generally better off paying in colones, and it's great to have small change on hand. The only reason why you should pay in dollars is if the price is in dollars. A lot of people will bring money to change or travelers checks to change, but you'll get the best rates by withdrawing cash in colones with your ATM card (every town has at least one ATM). Credit cards are also accepted at most hotels and stores.

More remote areas may be less apt to have restaurants and hotels equipped for credit card transactions.  This is due to the difficulty for businesses in collecting on credit card slips.  The merchant fees for credit cards are very high in Costa Rica from 7-9%.  Therefore, you will usually get better deals using cash or travelers checks.

If you really want to change money the old-fashioned way,  public banks such as Banco Nacional and Banco de Costa Rica have a better rate, but are usually slow and have long boring lines (but have lots of ATM machines, which are much faster). Private Banks such as Banco Cuscatlan, Banco de San Jose, Banco Banex are faster and give a better service.  HSBC aquired Banex several years ago.

Travelers checks are difficult to use in Costa Rica. Larger hotels *may* accept them, smaller hotels will almost certainly refuse. Although they previously cashed them, Banco de Costa Rica and Banco Nacional no longer cash them. Foreign banks may cash  them, although this seems to vary from branch to branch, and if so, you will be charged a hefty fee. Scotiabank, however, did exchange USD travellers checks with no fee as of August 2012, provided that you collect the funds in USD. Of course, if you exchange to colones, you will be charged a margin on the exchange rate.

Banks are typically open from 11:00 a.m. until 4:00 p.m. but try to visit in late morning to avoid before & after work and lunchtime rushes that can cause long lines. Your original passport is required - copies are not accepted.

Most banks still do not accept chip technology and are about 10 years behind the times. Bring Colones from your own country and use credit cards for purchases. Having to visit a bank, or even just an ATM, in Costa Rica can be a complicated effort. 

At the malls Banks are open until 7pm.