I have some dash buttons. You do NOT get S&S pricing with the button and lose the ability to shop around each time you purchase for the current best price. Therefore, I don't use them. Yes it's convenient to press the button but I'd rather go online, get S&S and see if a different sized package is cheaper or if there are any coupons etc.
BUT, these buttons are still good to get for .99 for a techie (and that cost can be offset by making 1 purchase. They can be hacked to do all sorts of cool things.
http://www.dashbuttondudes.com/blog/2015/12/11/26-amazon-dash-button-hacks Call an Uber, order anything from Amazon, control lights etc.
My idea is to have it order an uber and give feedback via lights that the uber is on his way for use when needing a taxi to get to the hospital when having a baby on Shabbos!!