one girl was like YOU HAVE BUTTONZ, I MUST HAZ! and didn't even look at anything but the buttons. :/
Yeaah that's my experience too. People (and by people, I mean female people) make a beeline to the button display, order some buttons and then walk away without looking at anything else. So, either they're only interested in buttons, and they wouldn't have visited my table at all were it not for buttons, or the buttons just narrowed their focus and once they had bought their buttons, they didn't want to look at anything else. I think some people visit every table once and try to find 1-2 things to buy from each table, so once they've found that 1-2 things, they move on never to return. And if that's your strategy as a customer, little $1 things are very appealing.
My assumption is that buttons *do* take away from my other sales, but I can't be totally sure about it. Next weekend might be the first time I go to a con without bringing my buttons, I shall either be very happy or very regretful that I didn't bring them...
Interestingly, I did have several people at San Japan do this: They would come by my table in the evening/afternoon or at the end of the con, and be like "Ohh I don't recognize this stuff, have we been here before? I don't think so... OH BUTTONS! Oh yeah I remember these! I bought this one and this one and this one." The fact that they recognized the buttons but couldn't even remember if they had been to my table until they saw the buttons, makes me think they probably ignored everything else at the table in favor of buttons =P