For brides, this is an economical purchase because it is convenient and it doesn't put a dent in your wedding budget. This could also be something that can be provided for guests of the wedding if the wedding is not that large so that everyone can continue to dance the night away.
I do know that they are sold in Delaware Rite Aid Pharmacy stores.
I have not purchased this product yet but I have reviewed some feedback and it seems as though folks are discouraged because once you change your shoes from heels to the flats, the pouch the flats come in do not hold your heels so you still have to carry around your heels.
There is an additional product that was suggested as well footzyroll: www.footzyroll.com
This product is similar to fast flats but it provides a large enough pouch to hold heels.
There is a cost difference $9.99 (Fast Flats) versus $25.00 (Footzyroll)
Both of these are cool ideas and should be thought of, for not only a wedding but for those weekend all-nighters.
Check it out and let me know your thoughts!