(I realise this is a bit dweeby, but I'm new to London and it's the first PH thing I've seen. Actually, it's the first PH thing I've ever seen full stop)
Never seen one out on the road, but have seen one at an endurance event that I was in a couple of years back (I think) - and I had mine on at the same time!
On my last two laps I stopped at the stream on both crossings to wash the mud out of the gears as they stopped operating. Still, it was mils better than mayhem the year before...
Indeed, they had to close of one of the sections as it got so muddy, great course though. Last year's was pretty crap in comparison, not helped by the fact that I snapped my chainring after 2 hours!