Well, it depends on the "plastic", and how it's applied.
I had a pair of Minolta Maxxum 9xi bodies for the better part of a decade. The 9xi body utilized a stainless-steel mirror box assembly, mated with a polycarbonate film chamber, cast-zinc bottom, and what Minolta describes as a "fiberglass-reinforced polycarbonate top plate with a scratch-resistant UV coating."
All I knew was that the thing was damn near bulletproof. And it really was hard to scratch. It took its bounces in seemingly good humor.
That, and its ridiculously-spec'd shutter (1/12800-sec? Well, not quite, but pretty close, according to both Modern and Pop Photo at the time they reviewed it in late 1992). The camera was quite solid in my hands, hardly "plasticky." While I'm a metal adherent, that camera showed me that it's not just what you do, but how you do it. Plastic doesn't always mean "cheap." (I think someone at Canon once pointed out that the metal/poly shell formulation on the early EOS-1 bodies was more expensive to work out than a cast-metal formulation would have been, but they were going the distance for the sake of increased durability. Yep, you read that right. (Of course, they moved on to cast magnesium for subsequent EOS-1 film and digital bodies, but you get the point.)
So, I prefer ferrous materials emotionally, but you won't find me dissing plastic out of hand. (Okay, not much.)
- Barrett