Like Nick I've driven over it a few times - but it's not very interesting to drive across. From the road itself you get very little sense of the vertical drop beneath the wheels.
I also cycled down the Tam a couple of times - once when it was being built and once since. From the floor of the valley it looks much more impressive - oddly enough more so during construction when huge sections were just jutting out into space like the highest diving board in the world.
