I was very happy with my choice in Cartagena. There are not a lot of truly budget options, but this is a good one. It is right in the walled, old city and is walking distance to everything (everything is walking distance to everything, really). The hostel itself is very nice; all blue and white with two small courtyard pools (one with fish, one for dipping into). There are a coup,d of hammocks and a nice living room area for hanging out, using the computer or watching tv. There is also a big table for eating or hanging around at.
I stayed in the private room with the shared bathroom. The room was ok. It is nothing special - just two beds pushed together, but it was clean and had good AC. There are 2 bathrooms: one for boys and one for girls. The bathroom didn't feel super clean, but it was fine. The shower was nice. The kitchen also didn't feel super clean, but i only used it for storing stuff in the fridge. (I should say, it may not be that anything lacked for cleanliness, but maybe things were just a bit shabby. Either way, it wasn't a problem.)
By the way, the room with the private bathroom looked really nice. if i was going back, i would stay in that room.
There was a computer for use and free wifi. There was always someone on hand to chat with or help arrange travel plans. The people that ran/worked at the place were so nice and welcoming. That was probably he best part.
It's not the best place i have stayed, but i think that for Cartagena, if money is a concern, this is a great option.