It's as big of an issue on my OSX touchpad devices, as I can scroll sideways with the same gesture I use to scroll vertically. But on any device where my primary HCI device is a mouse, it requires finding the horizontal scrollbar and click-dragging it. It's also not typically what people expect out of a website outside of the touch realm, which breaks a pretty primary rule in UI design (don't present the user with something they don't expect).