In an event situation, coordinators (i.e. the people who hire you) like to see elements of creativity like this. I've certainly done this type of shot before at galas and what not. It's a great way to include people's faces without interrupting them and getting the cheesy hand shake shot that you always see at these kind of functions.

My suggestion for next time is don't worry about the heads for reference, fill the frame with reflection. Also, if the available light is good enough, shoot it with the flash on and one with it off or if it must be with the flash then shoot the mirror at an angle where the glare on the reflection won't be so harsh. The real distraction for me on this one is the glare around the edges of the mirror where the cleaning hasn't been as precise. Not your fault, but it detracts from the image.