I preferred it over CO2 for engine room fixed extinguishing systems but when it contact with flame it gives off phosgene. That definitely doesn't do you any good!
When I worked for an airline in the eighties (before health and safety was invented) we had to attend fire fighting courses run by the local fire brigade and they didn't mess around. The small green BCF extinguishers that we carried were used against 8' x 6' vertically mounted corrugated sheets with petrol running down them and against large pans of water with petrol on top. We also had to don full smoke kits and haul live and kicking firemen out of an old Bristol Britannia fuselage as they struggled to resist our efforts.
The overriding memory of that training was in two lessons. Firstly never, ever aim a BCF at the base of the fire! The whole idea is to smother it and rob it of oxygen. Aim at the top and quickly work down to the base in a smothering motion. This also prevents the power of the jet causing curl over and engulfing you in flames. They demonstrated this by having a fireman kitted up in a full metal suit aim a BCF at a tray fire from about two metres away. The flames immediately curled over the jet and engulfed him head to toe in fire. It's an image that is still clear in my mind.
Also, as mentioned above. BCF in itself won't kill you but once it hits the flames it becomes about as breathable as tear gas. Again, to teach us the lesson we were all lined up downwind of a tray fire whilst a BCF was put into it. We were told to take one breath and then run like hell..........I never saw anyone stay for a second!
The main thing to remember about most of the extinguishers we use in GA is that they are there to rob an electrical fire of oxygen. If you release one in a sealed cabin you will know all about it so make sure you ventilate either with an open door or window as you let it off. A sealed canopy is more likely to result in your incapacitation long before the effects of the fire take effect.