Yep - It works just fine.
How do you wear it? I have Shure E4C's and when connected to the MPA I have an awful lot of wire to deal with, plus the E4C connecter is at right angles to the MPA socket...its just confusing me how to best wear this combination...
The newer Shure headphones have a connector where the cords from each earphone meet, if that makes sense. From there, they attach to a cable which can be swapped out which plugs into your device. With the SE series, you just swap out the bottom cord for an MPA.
I had E2Cs (I think) and switched to the SE210s just to deal with the long cord situation, but had someone who wanted my old earphones.
For you, I'm sure Shure would love to take $400 off your hands on some new SE420s...