Review (3/5): Fit is key...
This diffuser works fine if you have a hair dryer it fits, I have 3 hair dryers; it fit one of them - the Conair 1875, but not the 1875 Ion Shine. The latter has plastic projections on either side of the blower opening which don't allow a fit.
The "flexible" opening of the diffuser does not really have any give, so it fits or doesn't.