No, it's not an extreme sport, it's just fun. I have been unicycling for over eight years now, and riding off-road and trials for at least seven of those. While I wear a helmet, leg pads, and gloves, I consider the sport relatively safe (especially compared to any form of bicycling), as the top speed is <12mph, and a unicycle allows me to bail in any direction.
Pictured to the left is my primary (of two) unicycle. It is a 2007 Kris Holm unicycle with a custom carbon fiber seat (it's the only type that doesn't break regularly) and a few custom parts here and there.
In case your wondering, most unicycists don't appreciate associations with the circus. Unicycling, at least for me, is about focusing my mind, calming myself, and controlling my body, not performing for others. So please, no circus tunes.
A few pictures are below, to give you an idea of what one aspect of this sport is.