I do it every month, to a bunch of hard headed inmates.
These guys pull no punches, so I have to be fast-firm-and very focused.
If I am asked to preach, 20 mins is about as best we can hold their attention.
(they come of their own free will by the way to chapel).
I have in classroom situations - did a speech about abortion once in high school. But since then, no. I'm too passionate and would come across as an irrational nut. I'll leave the talking to those who appear more level-headed. That said, I wish I could.
I was asked to speak at the 2011 Australian Homebirth conference (that's what the photo is in case you didn't guess). I spoke for all of 15 minutes. It was scary!!!! But it went really well, and lots of people came up to me afterwards and thanked me etc.