I've been up there
Closed to bicycles for the most part as its quite narrow and gets a lot of tourist traffic. Amazing - just a spiral, one continuous curve.
We were in the Auvergne doing a motorbike test at Michelin's tyre-testing in Ladoux, near Claremont Ferrand. I had my Cannondale so I went along at the crack of dawn, bribed the guy at the toll gate with 30euros and he let me ride up. Incredible views over the volcano region.
When you come down you realise why they don't let bikes up there - DANGER! it's like descending a helter skelter only with cars coming up the other way, and you get to wear out just one side of your tyres
There's just no room at the top for something the size of the modern Tour entourage