We never had any problems and it is a friendly, pretty country the bit we saw, military presence in some places, but this was 8 years ago.
This seems like good advice from Lonely Planet website:
observations:
The FCO advice is essential if you are using UK travel insurance. You will not be covered by UK travel insurance if you travel to a “do not travel” area.
Medellin seemed safer than Quito and Guayaquil. Especially Guayaquil, which had metal bars across a lot of stores and security guards with shotguns outside. However, the day I rode the metrocable in Medellin, 2 people got shot while travelling on it…so, one still has to be careful.
Within cities (eg. Bogota, Medellin, Cartagena), there are safer areas and there are dodgier areas. Stay in the safer areas.
Bus travel: travel during the day. Take flights for longer distances.
Avoid gringo mistakes: talking loudly in English, conspicuous displays of wealth (in cars, public transport, on streets), and appearing lost. And, take care if going out at night in the cities…
Check out the travel guides (Lonely planet etc), their recommendations (generally) cover the safer areas of the country for travel.
Country towns are usually safer than the cities. Obviously, don’t go travelling to country towns in the middle of coca flower districts…
Keep informed: http://www.colombiareports.com – there are some articles which have a map which highlights which areas have recently had issues. Take care when relying on Colombians for security info – in my experience, most reply that the whole of their city or country is safe. Perhaps they are used to the unrest/dangers. However, if a Colombian says something is dangerous, listen closely!