Hello,
Last September I did my PADI Advanced Open Water with Andreas at Diving Planet in Cartegena, and my experience was positive. His English is very understandable (but not perfect). We did the class on Islas del Rosario, which, in my opinion was much nicer diving than the Taganga area. The reef was in beautiful shape out there, but of course it is more expensive, and watch out for the hotels trying to stick you with extra fees right before you leave, its a common scam out there. Get EVERYTHING in writing.
In Taganga I went with Calypso Dive Center for 2 fun dives, and I went with them because of price. I think me and a friend got 2 tanks plus all equipment for $45each USD. (at the time, 90,000 colombian). The price was the lowest, I think, because the equipment was kinda sketchy. But between my buddy and I, we had 1 working depth gauge (his) and 1 working air gauge (mine). We're both kinda laid back, so it wasnt really an issue, and we kept to 15'-20' for the latter half of the dive for security. Obviously Id recommend checking your rental equipment the morning of the dive, before it gets taken down to the water.
Also, its about this time of year when the coral does its reproduction. I was able to do the night dive to see that at Islas del Rosario, with Diving Planet, its a great dive, check into it when you arrive, maybe you get lucky like I did!