I've been using enviro cool (propane) for several years in 2 different cars and it has been fantastic. Next time I need to I'm going to use R152a, which is the dust can propellant. It has been tested quite a bit as a R134 replacement.

http://www.sae.org/events/aars/presentations/2008/manhoekim.pdf

Bottom line is that hydrocarbon refrigerants are more efficient and environmentally friendly than their fluorocarbon counterparts. If you are willing to accept the tiny risk of additional flammable gas in your vehicle then you are good to go.

Not trying to turn this into a refrigerant debate, it's your car you should do what you feel is best, just wanted to give you a personal experience point (I'm not selling the stuff).