I use this very shield and man, like the transitions glasses, they turn dark in a flash when in sunlight, turn clear when in cloudy low UV conditions. They work great, and you get spoiled wearing this shield on your helmet.
Alot of great deals on the the links above, I'm just very partial to Bell Helmets and the transitions shield.
I have a Bell Qualifier DLX in the Isle of Man theme, good helmet for $250 with the transition shield. I'll wear this helmet if the day is overcast and I don't need a full tint shield, I'll just let the transition shield adjust itself for the light. The shield by itself is very pricey, but if you buy the helmet with the transitions shield it is very reasonable. The Bell isn't as good as my HJC RPHA 11 Pro but it is less than 1/2 the price. Bell gives you a lot of helmet for the money.