bought a cheap replacement, it didn't work as it should and gave incorrect focal length info to the speedlight (which changes automatically from wide to normal when attached). Which is why I ordered several, so that the next time I misplace the diffuser cup I won't have to settle for copies.
What can I say, if you're shooting and don't want to set up your softboxes, this comes in handy. It does make a big difference with and without, as I've compared it. I'm sure most of you know this because it comes with the SB910 or 900