I have some firsthand and secondhand experience with these products. I've used both cryo-chem (http://www.cryochem.com
) and keep-it-cool (http://www.keep-it-kool.com
First, these products can be effective. In both cases they did an essentially perfect job of sealing the system for me. Anecdotally, they don't always work - I've read on various internet boards where some folks have had great success, others not much. Shoot, our own SteveBFL admitted his cheapskate business partner used it on a 140 chassis with surprising success. I'd guess (and this really is a guess) the success rate runs about 75%.
Second, these products are activated by moisture. The system must be extremely well dehydrated before installing them. This means changing the dryer and a very deep vacuum. If you can't pull a deep vacuum, they are not going to work satisfactorily. I personally prefer the cryochem solution; it's a two part solution which includes a drying agent. The sealer is added about 20 minutes after the dryer. Keep-it-cool is added all at once. Of course, cryochem is more expensive - $150 vs. $50.
I got lazy and didn't change the dryer one of the two times. Guess what - the sealer activated in the dryer, partially clogging it. Changing out the dryer followed by a thorough vacuum cured that problem. It isn't going to harm your compressor no matter what happens. It could clog the condenser, expansion valve, dryer, or evaporator. I just want to get through the Texas summer so I can replace the evaporator at my leisure later in the year.
AC shops hate this stuff, for two reasons. One is that it certainly robs them of revenue opportunities. The second is that it does occasionally activate in their recovery machines. But let me point out another side of that issue. The sealants are moisture activated. The only way they will trigger in a recovery machine is if the dryer in the recovery machine is saturated and beyond needing to be changed. Any AC shop with a "wet" recovery machine is pumping water into your system along with refrigerant, setting you up for an acid burnout down the road. I see a certain poetic justice in the whole thing.
My recommendations: 1) Only consider this if you do you own ac service work and won't be taking the car to a shop. 2) Changing the dryer, the markup on the sealant, etc. probably make it expensive enough that you should just go for the evaporator replacement if you have to hire it done. 3) Consider it a stopgap measure until you can fix it permanently. You'll still need to replace the vacuum flaps in the dash after using the sealant