I like the idea of those auto dimming helmets, pretty cool, but if you weld a lot, I've heard they're not the safest way to go.
I work in construction (management), and most of our welders won't use them. Although, a lot of the rookies do. One old guy told me he wouldn't use them because of the long term damage to his eyes.
It takes (approx) 1/500th of a second for the lense to dim after it senses the strike. That means for every 500 strikes, you have one full second of exposure to your eyes. Perhaps this is a bit anal, but makes sense to me.
With a bit of practice, the old head nod will bring your standard helmet down into position. Heck, it even looks cool if do it right