Hey Marty...

I actually used MDF to build the playfield of my home made "Vortex" pin project. I do think ply would've been better, as screws etc hold much better in plywood than they do in MDF i have found. Having said that, the playfield is as good now as the day it was made nearly 15 years ago when i built it.
The main reason i tried MDF was that the MDF had a nice smooth flat surface that didn't need much preparation for paint etc, it was cheaper, and that i had problems finding decent plywood at the time.