I have two pairs of MOP I got from Carlo Franco. I use them with most of my shirts. They don't quite fit some of the shirts I own (in which case I use the wide, more firm Borelli stays), but the majority of them they work quite well. They are thick enough that they won't break, but don't feel like weapons into my jawbone like metal ones do.
I, too, have several that I got from Chuck & Jill. Great stays, and they fit most (but not all) of my shirts. I definitely like these stays better than the brass and sterling stays that I have (as these tend to bend over time...). And for what C & J sell these babies for, they're good value for money (relative to what I've paid for MOP stays at places like Pink, etc.).