My feeling on these is that, they're cute and fun, they certainly can't hurt, and they probably help -- just not very much.
All the pictures people are posting really don't signify -- reflective accents _always_ look awesome in a camera flash. Out in the real world, they never look as good. If you're right directly in the path of a car's headlights, they can look almost as good, but you can't always count on being directly in the path of a car's headlights -- especially from the side: by the time you're directly in front of a car that's coming at you from the side, either it's far enough away that you'll be out of its path before it can hurt you, or you're being T-boned before it's possible for the driver to react.
If you really want to be visible, you need active lighting, rather than passive, and I think this is extra true for being seen from the side. I run blinkies on the sides of my rear rack (as well as on the back of the rack and the helmet, of course):
Additionally, as others have said, tires do tend to get dirty fast, so unless you keep the sidewalls meticulously clean, they're even less effective than normal reflective accents.
Bottom line: I like that my tires have reflective sidewalls, but I would not go out of my way to buy tires with them; and I never trust them (or any other reflectors) to make me visible, I rely strictly on active lighting for that.