I have Smith I/O goggles. I second those who've recommended the Smith Blue Sensor Mirror (70% VLT) as a low-light lens. For "mid-light" I have the Igniter Mirror (35% VLT), and for big-sun days I use the Green Sol-X Mirror (15% VLT). Having three lenses sounds like overkill, but I pick one to put in the goggle frames in the morning and carry the one "next" to it, in case the light changes much during the day.
Now, a partial thread hijack: I don't really like carrying a lens in just the cloth bag and sticking it in my pocket. It's just a matter of time before I fall on it, or someone whacks me in the lift line, and the lens cracks. Has anyone seen a single lens case that is small enough to fit in a ski jacket pocket (or camelback pocket) and offer some impact protection for the lens?
If you carry a lens in a case, you might as well carry a spare goggle. It's faster to switch too. That's why I think that interchangeable lens googles are only good for travel. Otherwise there is no real advantage over having two goggles, one for bluebird, another for a storm day.