I have found that the cause of death of most of my CFLs has been cheap capacitors that fail prematurely. This has usually caused too much heat buildup in either the capacitors of the FETs they are buffering, leading to a catastrophic (read, sparks and smoke) failure. As a result, if there is a question about the condition of a bulb, the first thing I do is give it the "smell test". If it doesn't smell like charred electronics, there's a decent chance it works.