Any opinions....

Can anyone tell me if they have any opinions about the white balance lens cap? I saw it on photojojo and decided to google it. I found this review.

White Balance Lens Cap Review

I did a search on ebay and found a lot for <$10. There are visual examples and results on the review, but that's just one review and I was wondering if anyone else has used this or a similar product. If so, what are your feelings on it? I know it's only <$10 but it's still good to have some feedback from others.