Some time ago a neighbour lent me his Canon 5D Mk II full frame camera for an afternoon. I was curious to see the difference in image quality between that and my crop camera.
I took several photos of birds in my neighbourhood and when I viewed them on my computer found that after the usual post processing I couldn't see too much difference. In fact, if anything, I felt the images from the crop camera looked slightly sharper and resolved. The full frame shots were 'smoother' looking. Two different 'looks' depending on what you like.
However, where the full frame image quality came into its own was when you magnified the photo. The Little Corella posted above was taken with the 5D. If I zoomed the full size image so that just the head and shoulder filled my 27" screen the detail and quality is extraordinary. Every tiny crack in the bird's bill, every little feather around the eye, even the blue skin around the eye - reveals wonderful detail and not a sign of pixilation.
And this is all on a creamy smooth image. I can imagine you can enlarge these full frame shots to poster size and they would look terrific.