OK, I'll toss out a success story. I used it to fix an evaporator leak on my 1987 124 wagon. Car had a two-month kind of leak - a/c would work fine for one month, barely cool for a second month, and was after that essentially useless.
I used the cryo-chem brand sealant. Worked perfectly. A little bit of inital leakage while it did its job, then perfect. System cooled great all through our Texas summer.
About a year after installing the sealant the front seal on the (old, remanufactured) compressor failed, dumping the oil and refrigerant charge. The failure did not cause any problems with the sealant.
It is very important the system be thoroughly dehydrated when using one of these sealers - they are moisture activated. Perhaps the problems seen by techs here occurs when the sealer is installed into an R-134a system filled with "wet" PAG oil. (PAG oil is extremely hygroscopic.) In my case my R-12 system was filled with mineral oil - much easier to eliminate moisture.
My system is currently back up & running just fine, but without sealer. I had to disassemble & flush because it suffered the "black death" when the compressor failed. I did not see any signs of the sealer inadvertently activating inside the system.