Frequently. At least, within a season. Size guides for outdoor clothing are generally accurate, including the run small/run large meters. For non-technical clothing I will shop online for brands I know fit me well, such as Uniqlo. I buy a fair amount of my technical clothing online because the stores don't always carry the brand or type of clothing I'm looking for. Otherwise, it's because it will save me a trip which could be out of the way.