In the past a number of posters have said nice things about Colombia. Big problems in DR: lack of 24 hour electricity service, corrupt politicians. I think it is hurt by being an island and having to import everything. Have not been to Columbia but its hi on my list
Medillen is dangerous for extranjeros. If you can't pass as a local DON'T GO! Kidnappings and robberies are rampant. But it is a pretty area. If you go stick to Bogata or Cartejena. Bogata is just another big city, I have worked there a lot. But in general be very carefull.