I don't know any statistics but I certainly think people find them very useful where they are in London, (Holborn for one).
It gives people a chance to rethink dashing over the road at the last moment, as they can really see how long that last moment is going to be (though there will be a slight delay after the timer has finished I'm sure.
They show a numeric countdown (15 seconds if I remember correctly)
Still it doesn't help stop all the London drivers who seem to think that it's the slower party who should always get out of the way, rather than they that should slow down or even stop for an obstruction that is right in front of them.
But overall I think they've probably prevented a lot of accidents.