Envirosafe is a mixture of propane and butane. I haven't used it myself -- I choose to convert to HFC 134. But the buzz on the internet is that it's a very good refrigerant... cools well and mixes well with mineral oil (r12 lubricant).
Some automotive people are quick to point out that the 8 oz. mixture is flammable, yet choose to ignore the fact that an automobile can carry up to 20 gallons of highly flammable liquid.
The down side is that should a leak develop, the leak rate for the two components - propane and butane - are different. Plus, I believe it's banned in 17? states. This also means it's legal in many other states!
As with any alternate refrigerant, one has to worry about the long-term effects of the refrigerant on the a/c system.
There's a lot of information on the internet about Envirosafe. Start by looking in aircondition.com.
95 E320 Cabriolet, 131K