I've only been to Medellin, but I loved it there. And based upon my experiences there, I came away with a very favorable impression of the country and the people.
How does Colombia compare to the DR. Well, as someone has already noted, Medellin is way ahead of any city in the DR as far as infrastructure is concerned. And one of the main differences I saw, even in some of the seedier parts, was how much cleaner Medellin is than SD or Santiago. The entire time I was there, I never saw anyone throwing trash on the ground or out of their car window. And the city itself is beautiful. There is a point during the drive from the airport to the city where you come out of the mountains and are suddenly treated to a breathtaking view of the city in the valley below you. The view was so awesome that I had my driver stop so I could take pictures. An absolutely gorgeous city.
Can't speak for the more touristy cities like Cartagena, as I've never been there. But, as AZB noted, I would advise anyone who is not already fluent to serious bone up on their Spanish before visiting a city like Medellin. I met only one or two other english speakers the entire time I was there. And I had to dig really deep into my spanish vocabulary to keep up with the locals. Having said that, like RacerX, I found Colombians easier to understand. But I honestly believe that it's solely because Colombians speak a lot slower than Dominicans do.
Property seems to be cheaper there. I rented an apartment in Poblado, a very upscale section of Medellin. And I did a little research while I was there. Luxury apartments in brand new highrise developments in Poblado are selling for less than $100k USD. I'd be hard pressed to find a luxury highrise apartment in an exclusive neighborhood in SD or Santiago for that little.
But one thing that Dominicans do better than Colombians is party. The DR's music, dancing, and nightlife blows Colombia out of the water as far as I'm concerned. I love the fact that you can go just about anywhere in the DR and find a live band, and a spot for dancing. Colombians are festive, but they can't party like Dominicans. In fact, Colombians have begun to borrow from Dominican music and culture to enhance their own. Bachata is very big there. And they've developed their own variation called Bachanato (sp?). Which sounds just like bachata to me. But the locals swear that it's different.
I guess I love things about both countries. When I want to relax and enjoy the city, I choose Medellin. But when I want to have fun and party, and/or enjoy the tropical sun, give me SD or Santiago.
Vince.