There have been comments of around 12 secs of useful consciousness after an explosive decompression. Ignoring all the other chaos in the cockpit for a moment ( noise, debris, panic, cold etc ) why would pilots lose consciousness so quickly when people can swim underwater for much longer than this.
Because of the O2-Hb dissociation curve. Put simply, if the partial pressure of gaseous O2 in your environment is lower than that effective in your blood, Haemoglobin will release O2 in your lungs instead of absorbing it - effectively, you are breathing 'out' O2 instead of breathing it in.
At least that's how I remember it from AvMed trg...