Thanks everyone for their answers.

I'm glad to hear that even the inexpensive models do the job they're
intended to do.

Although having been blessed with excellent eysight for most of my
life, I've now passed that stage where I have to hold the book further
and further from my eyes to achieve focus, found that my arms simply
aren't long enough no matter how far I stretch, and that I need
reading glasses. So at the very least I think I'll get a helmet that
allows the use of a magnifying lens. Variable shade control sounds
awful useful too, for related reasons. This may rule out the bargain
models, but I can live with that. I'd rather spring for high quality
in the beginning and not kick myself later for false economies.

Mickey