I'll answer your question, but I hope we don't get in trouble for not being DR related! I visited Cartagena, Santa Marta and Medellin. Liked them all, but the coastal towns are very hot compared to the north coast of the DR. Medellin has a cosmopolitan feel and cooler climate but after doing a city tour not much to do apart from the nightlife. Others will probably feel differently. Overall, I won't be moving from the DR.
By the way, there was a thread about the CheckMig entry and exit forms and it not working well online. I just completed what I could and that was enough for the brief glance they received.