They are not affected by temperature, I ride anywhere from 0-110* F and have never had any problems. It is also plenty dark enough for me. I swear by the transitions shields, I'll never use another.
Significant rock chips will cause the area where the chip hit to not transition anymore of course, my current screen has about 2,200 miles on it and has just one such chip. Time also degrades the transitions, I replace my shield about every 15,000 miles. After that the shield does not go as dark, and takes longer to transition.
My only complaint on the shield is in overcast/rainy days. Since it is UV activated, on overcast or rainy days it runs much darker than it should. I generally swap out my clear shield when it is overcast outside.