Duracool is a hydrocarbon refrigerant. Typically these refrigerants are a blend of propane, ethane and butane. A hydrocarbon refrigerant has greater heat capacity and will cool better than R12. It does not react with moisture to form destructive acids in the AC system. It breaks down relatively quickly when released into the atmosphere and has no long term degrading effect on it. With these advantages one would think the automotive industry should have started using this years ago. Problem is that it is potentially explosive. Of course you have a 20 gallon tank of liquid fuel in your vehicle which is highly explosive also. But it does not get routed through the ventilation system in the manner a refrigerant moves through it. Most US states have laws banning the sale of hydrocarbon refrigerants in their borders. But the largest drawback from a practical standpoint is that it is a blend. The components tend to fractionate/separate at different rates in the system if there is a leak. So the proportional blend changes and you cannot simply add a little of these refrigerants. The proportional blend ratio is critical in maintaining cooling effectiveness. The system has to be evacuated, and a fresh supply in the correct proportions installed when there is a slight leak. That would be a nuisance. Hydrocarbon gases are odorless. Mercaptans are added before final distribution so that if they leak it can be detected through your sense of smell. Mercaptans are sulphur based products. If you have natural gas(basically 100% methane) or a propane gas grill around your home, if it is leaking and not being burned you notice a rotten egg odor. That is the sulphur in the mercaptans.
Wow, acarl gave a good detailed answer. I'll just add the following: I live in hot North Carolina and am using R134a in a rebuilt stock system with *no* problems -- plenty cool and with R134a, I don't have to worry about any confusion on the part of the AC guys when maintenance comes due.