When I worked with the high schoolers this past summer, I'd estimate that 75% of them had a FCUK shirt. Of course, they loved them.
I didn't think they sold them outside of French Connection stores, but I ran into a few at the Virgin Megastore in Union Square last week. I don't wear FCUK, but I have a French Connection turtleneck sweater (no printed T-shirts for me!) that I love.