The EOS 50D is the latest incarnation of a product line that has always reliably delivered on image quality and you won't find any nasty surprises in this review either. The 50D delivers a lot of detail and clean images with well balanced contrast and colors that leave some latitude for 'customization' in post processing. Canon's standard-across-the-range Picture Style tone and color combination also produce consistent performance between models, which is useful if you're moving up from a 'smaller' model.
Having said that, in terms of detail the 50D is not quite the step up from the 40D that we would have expected. After all the new model's nominal resolution has increased by approximately 22% in both dimensions. There is only a very small amount of extra detail in the 50D output though (in fact even at 100%, if you scale the 40D's output up to match the 50D the results are almost indistinguishable). While the new sensor makes the 50D the highest megapixel APS-C DSLR currently on the market it also makes it the one with the highest pixel density and it appears that Canon has reached the limit of what is sensible, in terms of megapixels, on an APS-C sensor (using current technology). At a pixel density of 4.5 MP/cm² (40D: 3.1 MP/cm², 1Ds MkIII: 2.4 MP/cm²) the lens becomes the limiting factor. Even the sharpest primes at optimal apertures cannot (at least on the edges of the frame) satisfy the 15.1 megapixel sensor's hunger for resolution. The result is images that look comparatively soft at a pixel level and only show marginally more detail than images from a good ten or twelve megapixel DSLR. If all you end up with is a larger image (and file) one starts to wonder what the whole point of pushing the resolution up to these dizzying heights is.
Considering the disadvantages that come with higher pixel densities such as diffraction issues, increased sensitivity towards camera shake, reduced high ISO performance and the need to store, move and process larger amounts of data, one could be forgiven for coming to the conclusion that at this point the megapixel race should probably stop.
Just to make it clear, the 50D's image quality is (at identical viewing size) and by no means worse than the competition's but it's also not significantly better than the 40D's (Dynamic range and high ISO performance are even slightly worse) and that simply makes one wonder if the EOS 50D would have been an (even) better camera if its sensor had a slightly more moderate resolution.