I hate wearing my glasses to ride. I lose all the peripheral vision I have when I have contacts, like what Grog described - I'm also very nearsighted.

It's worst when I am trying to glance over my shoulder to look for traffic, etc. Can't see a thing unless I dramatically turn my head so I can look through the glasses. I've only had to wear glasses a few times, after scratching my cornea for example, so I have been lucky.