Not being too experienced with exchange rates in D.R., but having gone three times since July of '03, I must say that the all-inclusive hotel rates are way below anything else you see.
And I went to a few Casas de Cambios, and they all told me a lot less than even the bank was giving. Actually the rate right at the airport I found to be pretty nice.
What I did find was that by using my ATM card to retrieve cash (pesos), upon my return home and checking my bank statement, that my exchange rate was extremely favorable. I never actually calculated it, but if I thought I took out the equivalent of $60 USD, I ended up seeing maybe $52 or something like that on my bank statement, as an example.
Also taking small dollars for tips and the like is a lesson I have learned. It's easier to give someone the exact USD amount, and not have to deal with the exchange rate when getting your change. Or to give the bellman $3 or $4 USD.. Personally I get confused all the time as to how much I'm actually giving him. Sometimes we think, oh I have him 50, but 50 pesos is not the same as $50 USD... so we end up short-changing those who labor so hard for our comfort.