I mean how strong are they really and how much do they affect the vision?
They're surprisingly strong. I saw one as pax when OS457 was targeted on approach to LHR 27L last night. Despite being side on, and thus being much harder to track the aircraft, the glare was incredible and although I experienced no visual sensation it was most uncomfortable. I can only imagine what it must be like seeing these things nose-on from the flight deck where it's easier for the muppets to aim.
(Un)fortunately, I (and I suspect most of the other passengers) got quite a good look
at the location of the laser
here.