With my husband, and with casual playpartners, it's all about the green yellow and red. I use green a lot cause I, for one, love knowing when "all is well". Those are more feedback words to me than safewords - I want them respected, of course, and will stop playing with anyone who doesn't respect them. And when I top I always suggest the G/Y/R system to whomever I'm topping.
My actual safewords, in order of severity, are "FAINTING", "VOMIT", "LAWSUIT"