Andrew M - [non-medical opinion to follow]
I'm sure that I've seen negative opinion on photochromatic lenses on PPRuNe and a search reveals that this thread has a posting by
Basil on 20 Apr 03 - 2129 which indicates that they are not recommended.
My experience from using them for driving is that they don't react quickly enough to eliminate glare when the sun suddenly appears (from behind cloud, turning, coming out of tunnel, etc) and similarly going the other way. I realise that the argument applies for if you have non-photochromatic lenses (ie) sunglasses / clear lenses, but you tend to expect the lens to react and wait until it starts to change, accepting reduced visibility / eyestrain for a longer period than if you just whip out the spare pair. An optician once said to me that photochromatic lenses interfere with the dilation / contraction of the pupils and could have negative impact in the longer term.
Would recommend either having flip up / down clip-ons or a spearate tinted pair of spectacles.