I'm heading up north to Cartagena later today on an uber-cheap 45 euro flight with a company called Aires ($COP 114,000)! I can't get over how cheap Colombia is.
I'm flying as my time is limited, but generally speaking it's advisable to fly between destinations where possible as the risk of kidnapping/robbery rises considerably once in rural Colombia.
You would think that after 3 days I would be BETTER adapted to the altitude in Bogota (2,640 m), but I seem to be finding it harder and harder to breath or partake of "strenuous" activity like walking the streets! Maybe it takes about 4-5 days to really acclimatise, but it's quite a remarkable feeling.
I thought for a while that I must be getting really unfit from eating all the starchy Colmbian food and mucho cerveca! 
Anyway, I got a good feel for Bogota now (initially viewed as a tad rundown, but then I found the hidden gems), and feel ready to move on.
(Added in March...)
Just want to add a quick note after having experienced other cities in SA since Bogota. At night, this was the one city I really DID feel like I could potentially get robbed as there were countless times I had to cross the street to avoid unsavory characters in my direct path. It didn't happen mind, but I just had that feeling particularly strongly here. Only at night. Daytime I felt perfectly safe, but take obvious precautions when using the buses etc (keep bag in front of you, zip up pockets if possible, etc)
..later after arriving at Cartagena..
The guys on the flight...
On arriving in Cartagena on a damn nice Aires flight, I shared a taxi with Rob and Ignacia, two fellow backpackers who were on my flight. Ignacia is from Chile and Rob nary speaks a word of Spanish (despite planning to travel S America for 12 months) so it was damn handy having her around. Coincidentally she was staying at the same hotel I was in, Hotel Toledo, which at 17 euro for a private room, is the cheapest such option in Cartagena. We later on hooked up with a guy called Mike who was staying at the same hostel as Rob (and who was ALSO on our flight!) and we all went out for food and mucho alcohol.
Ignacia knew all the cool places to go as she had been here before.
OK one of the first things that hits you about Cartagena is the price of everything. It's REALLY expensive by Colombian standards! It's the prime destination for all holidaying Colombians, so this is probably why. Designer shops such as Tommy Hilfiger line the street, and meals average around 30-40 euro in the old quarter of the city.
We had a few nice mojitos in a place called Café del Mar, which is located at the top of one of the forts surrounding the city, sitting beside the ancient cannons. The night went on late and much revelry 'twere had!