I have had two rapid (as opposed to explosive) decompressions on two unrelated aircraft types. In both cases, surprisingly, the root cause was essentially the same - the aircraft decided (through squat switch logic and related fault conditions) that it was on the ground, and therefore dumped the cabin.
The good news is that I can tell you that decompression doesn't hurt - usual drills calmly executed, etc. Unfortunately, on the first aircraft mentioned, the "I'm on the ground" fault was intermittent. The bad news is that when we were a few seconds into the drills, the aircraft changed its' mind again and decided that it was airborne after all, at which point it re-pressurised with a "whump" - and that hurts like hell!