I ride on them every day on my commute, and when they are wet they are damn slippery, and dependign on the layout of the stones it's quite possible to get a tire stuck between them.

That being said, I have an acquaintance who is a former pro rider who rode Paris-Roubaix and once commented to me that his SAG wagon guy was a former rally car navigator who said Paris-Roubaix was the worst road he'd ever ridden on.