In reply to malc:
I've never had a problem with my glasses not staying on my face when running, even on rough, off-road runs. Light (frameless?) lenses help, though I know that for strong prescriptions these can be very expensive.
Rain and misting can be a bit of a problem occasionally, and heavy rain in the dark is a serious issue. I've just started wearing a cap when it's wet, which helps.
For fell races and long road races I do tend to put contacts in though. My vision isn't as good, but it's good enough. I tend to make large-print maps for fell races.