Very hard. A tribute to Flanders, Chris, Stephen and I took a ride to a true Muur. 20%, cobbled and right here in San Francisco! Chris was first up, zigging and zagging at the top, but up nontheless. Stephen up next, and next thing Stephen down making kissy faces with the ground again. Oh boy, I'm next. Hit the Muur hard, after the intial momentum faded, I had trouble controlling my bike, my weight made itself to the back - the way back, and I thought I was going to make kissy faces with the ground. One more pedal turn, unclip, then walk. After the cobbles, the climb is not over - hop back on the bike and keep pedaling.
How hard was it? Don't take my word for it, try it yourself -