I'd be willing to bet that most people don't encounter horizontal scrolling on a regular basis. At least I sure hope not; it's hella annoying. The only cases where I can think of it coming up (for a full page, at least) is when there is an over-sized image or on the occasional artsy site where it's designed that way intentionally. Perhaps horizontal scrolling comes up more often for people using small screens--on a netbook or phone or something--but even then I imagine it's fairly rare.
The use case? Inline comments, feedback boxes, contact info, control panels, really anything you want to have immediately available but out of the way most of the time. Is this the best way to implement those? Not always, of course, but probably enough to warrant its existence.
Oh I hate horizontal scrolling, but it seems odd that the contents of the page are shifted to the left in the demo without any clear way to shift back. Maybe I missed something, but that's usually an indication of a poor UI paradigm. And I'm not arguing it shouldn't exist, just commenting that it's usefulness isn't immediately obvious. More choices are always good though!
Ah, that's what you meant. You just have to click on the original page to shift it back. It was intuitive to me, but I can definitely see people not realizing it. You're right, they should definitely indicate how to get back in some way.
It's really quite an awesome way to show more content. A lightbox works too, but is not well suited for content with more vertical length then width.
IMO, it's unacceptable for a user to have to scroll horizontally in the first place. In fact, it's probably one of the most important usability constraints.
