bromochlorodiflouromethane (Halon 1211) is extremely dangerous. It uses a chemical reaction to remove oxygen. You're only supposed to use it in an extremely well ventilated area, or with an air supply (ie SCBA).
It can cause extreme light headedness and nausia, a severe drop in your oxygen saturation (sp02), and hypoxia.
The biggest positive is that it's a "clean" fire fighting agent, which means it will not damage electrical equipment.
I would recommend PKP (potassium bicarbonate) over halon, but it is corrosive to soft metals and must be cleaned off well after use. It's EXTREMELY effective against fuel and electrical fires though.
If you stick with your halon, make sure doors are opened, or windows are down... it's really pretty dangerous to breath in.
It would be very good for your engine bay though... Won't ruin electrical or corrode the wiring or aluminum block. Just be careful!!!