I've never had to use a fire extinguisher in a helicopter but a friend of mine had a dry chemical fire extinguisher discharge in a helicopter once. It was in an AS350 and it was discharged accidentally by a patient for only a brief period. The dry chemical agent proved to cause a very large distraction to the pilot as it was discharged but his situation worsened once it started sticking to the windscreen. He was forced to use the small sliding window on his door as his primary reference in poor weather in the mountains. Everything worked out fine but it doesn't sound like a situation I'd like to repeat.