Photochromatic lenses have been around for many years. In the early days they were available in glass only and then went to plastic a number of years ago. These are lenses that change and get dark in the sun and then change back when one comes back inside. The vast majority of these lenses are now called transition lenses and are available in most materials today. The newest generation of transition lenses came out about a year ago and are FANTASTIC! They get much darker than previous generations and change more quickly than earlier generations. These lenses also block harmful UVA and UVB rays that can cause damage to the eye over time. All of these lenses in this "photochromatic" category are temperature dependent. That is they will get darker in cold weather and not as dark in warm weather. Also inside of a car the windshield will block many of the rays that make these lenses change and get dark. Therefore for driving a dedicated sunglass is a better choice. But for general use you will find transition lenses a beneficial feature to add when you are getting your new glasses. This is something that must be done at the time that your glasses are ordered and cannot be ' added on ' at a later date. This is because the material is different to allow these lenses to change and get light or dark.
If you had an older generation of these types of lenses in the past and were not satisfied, do yourself a favor and try this new generation of transitions. You will be glad that you did.