Prescription glasses that self darken

I wear prescription glasses fulltime and have always found it annoying having to change to prescription sunglasses when required, resulting in having to carry around two pairs of glasses and so on. Not much fun.

My friends told me to get prescription lenses that will self darken when in sunlight, and then fade back to normal when out of the sunlight. These are called Photochromic lenses.

Are these any good at all? Do you know where I can get discount prescription glasses with Photochromic lenses? How quickly do they fade in and out?