I have never (yet) flown in a GA airplane with a flat panel PFD/MFD.
I'll bet many in this forum have not either - but we all will very soon,
because the proliferation of such systems has far outpaced even the most
optimistic anticipations of only a short time ago. It has also outpaced
the
tradition accrual of collective experience, with the gradual adoption of
newer
technology. While the overall improvement in situational awareness with
such
systems appears incontestable, I wonder if anyone is concerned about a
risk of
degraded scan technique while staring at a single flat panel for most of
our
information. Isn't there a risk of a "glassy-eyed" stare - particularly
when
we're a bit tired - slowing or degrading the learned uptake and processing
of
information?

No, not at all. You still have to move your eyes around, and you'll want to
look at your MFD too.