It really depends on what type of lens you get. there are lots of different levels of mirrored lenses that let in more or less light.
I personally own the Oakley Airbrake goggles and they came with two lenses (fire iridium for the day, and a high intensity yellow with a bit of blue iridium for overcast/night). They both have a mirrored effect, but its much greater on the fire lens.
IMHO, its always best to have two lenses because one really can't cover you in all situations. There are a TON of models out now from different brands with quick change ability too!