I can probably count on one hand the number of times I've actually paid for FC, though I've been fortunate to have good upgrade luck. The only negative experiences I've had have come from other FC/elite passengers, who sometimes think I'm trying to board at the wrong time or have seated myself in some other more deserving soul's seat.
I'm youngish (not really anymore, I suppose!) and am usually wearing jeans and flipflops, hair still wet from the shower, blearily working my way through a hangover... I've been asked something along the lines of, 'Are you sure that's your seat?' more than a couple times. It's worked out well for me, though, because I reply with something like, "I know, guess they can't exactly accommodate window requests on upgrades... Don't suppose you'd like to switch?" and someone in the cabin always overhears and immediately offers to take my aisle for their window.
As for coach passengers, it's normally just a look of envy as they look at the big seats and then glance forward to coach - I figure I probably have a similar look when I'm trudging to the back of the bus.