We took only dollars, most prices are in pesos and dollars, but I don't think it makes a great deal of difference. I would reccomend buying your entrance visas over the net before you go to save queuing at the airport, and don't forget to save some cash for your exit tax, which is 20 dollars each iirc. Dominican is fab, and the people ate amongst the nicest I have ever met, I'm sure you will have an amazing time.
From the Foreign Office website on the Dominican Republic (under their travel advice section):-
The currency is the Dominican peso. American dollar and Travellers cheques are easily exchanged. Currency should be exchanged only at banks or official exchange offices (casas de cambio). UK debit and credit cards are widely accepted. Exercise caution when using debit/credit cards. There have been reports of fraudulent charges appearing after card usage in the Dominican Republic. Exercise caution and be aware of your surroundings when using an ATM card.