"Mine is more like a curve than a circle and, like you, that's all I get - no actual headache, although my head feels 'dull' or 'muzzy' for an hour or two..."

I could just as well have said curve rather than broken circle. By the time it extends to the periphery of my field of vision it's definitely more of a curve. I can be headachey before it happens and feel fuzzy for a while afterwards. I'm definitely not safe to drive for an hour or more.

The visual disturbance is a painless phenomenon while it's happening and I too have learned to relax, do nothing for half an hour and, well, sort of enjoy it!