Don't use extenders much now, but I found some old shots in the days before I got a Canon 300 f/4 IS. This was how I got some reach with what I had. Efective 400mm + reach...no IS...handheld
I use it all the time with my 200mm, because I usually use time priority, no need wide f-stop. extender is money when funds are low. love motorcycle scenes. heading out to Motegi first thing when the snow melts next year.