Here's maybe a slightly different viewpoint to the general consensus opinion of 'this is a bad idea':
Will adding a lens modifier- generally speaking a modifier of significantly lower quality than the lens itself- detract from the quality of a lenses optics? Yes. Absolutely. Your picture will never be as sharp or accurate as if you hadn't added the modifier to the front of the lens. In fact, it will be significantly degraded. Period.
Should you buy this item? That depends on what you're looking to accomplish with your photography, and what (in general) you want your pictures to look like. If you like the 'lo-fi' photography look that can be achieved through the use of things like Lensbabys, and Lomo cameras, then you're likely to think the look that comes from one of these modifiers is cool! If, on the other hand, you're hoping for performance that in any way rivals an actual dedicated zoom lens on a 'real' camera, it would be better to avoid such modifiers because the quality of the images you'll be able to make won't even be in the same city as a dedicated lens would, let alone the same ballpark.
Personally, I think this modifier looks like a heck of a lot of fun for use as a 'goof around' camera attachment, and at ~$35 it's a small enough investment that it seems like a bit of 'no-brainer' to give it a go...