Like everyone else I have had mixed success with it, depending wholly on the technology used at the store in question. Stores with old bottom glass only scanners are certainly a no go. The ones with bottom and side glass however do sometimes work, and of course wand styles work the best.
There is no way I would ever go back to carrying around a bunch of cards, heck even half of those were so worn that the cashier had to punch them in by hand anyway, so I don't get squeamish about them having an issue scanning it... they have issues with the real cards too.