I generally think of them as not being worth it. I've only been bike commuting for lass than two years now, so only one full winter, but I have lived in the area for 10 of the last 15 years or so. Given the weather conditions most years, I think I would miss on average one or two days a year due to ice. Given that, I think it's probably just easier to take metro for a couple days. Besides, I know as soon as I buy a set of studded tires, there will never be snow or ice in DC ever again.