It's no wonder that air conditioning pops up on the Saab problem list, because A.C. systems seem to be a problem on ALL cars nowadays.  So don't blame it on Saab, blame it on maintenance.  The most common failure is compressor failure.  Compressor replacement also requires dryer replacement and should include expansion valve replacement; evacuation and recharge is also required.   So be advised that if you're looking at a used Saab with inoperative AC, tell them you want it operational before you buy it or negotiate for $750 off the price; don't let them talk you into "it probably just needs freon", because what it probably needs is at least $750 worth of work.

Symptom: Profuse sweating when inside car in summertime with windows rolled up.

Preventive maintenance:  To give the A.C. system (and yourself) a fighting chance - Get it serviced every year!  

Have the AC checked every spring by someone who knows air conditioning systems.  Try to find someone who seems to make some kind of sense - probably the Saab dealer is the best bet, because he knows the nuances of the Saab system and I've run into plenty of characters who DON'T know the nuances of Saab systems- and see if you can establish some sense of trust over the years.  Do it every year.  This will probably detect small leaks before they become a big problem.  When a system leaks all its contents out, it replaces the normal contents with air, and that causes corrosion; that causes the whole system to deteriorate and it's expensive to fix.  Not only that, it will be more prone to fail again.  The key to maintaining AC health is to catch little problems before they can become big problems.

Another thing you can do is make sure the cooling system is in good shape.  Make sure the fans come on when they should (there's one on the front of the radiator - on a '91, anyway - that should come on any time that the compressor is activated).  Fan failures can lead to high AC pressures.  Again, this is where it makes some sense to have a Saab pro looking at it.

Have a pro do the work.  This is not a DIY job.  If you don't have a thorough understanding of AC systems and the necessary gauges and equipment (and by definition you don't, because if you have that stuff, you're an AC pro), then take it to a pro.  The one exception I've seen is Robert "Kappy" Kaplan, a lawyer who taught himself air conditioning.  Now that, folks, is one smart lawyer.