With entry-level dSLRs getting pretty cheap and close to commoditized, competition for the attention of experienced amateur photographers is heating up the $1,000-$1,500 price range of the market. Former occupants of that segment, like the Canon EOS 40D, have dropped to entry level, posing their own competitive threat to newer, more expensive models. The meat-and-potatoes updates the EOS 50D offers over the 40D--higher resolution, one usable extra stop of sensitivity, modest single-shot performance improvements, and multiple compressed raw options--provide a compelling alternative. But it's missing the vegetables, like an improved AF system, smaller spot meter, better viewfinder coverage, and customizable boundaries for shutter speed and aperture, which might have pushed it from compelling to must have.
Canon offers three configurations of the 50D. One kit includes the veteran f/3.5-5.6, 28-135mm IS USM lens, with an angle of view equivalent to that of a 44.8-216mm lens on a 35mm camera, and a second kit comes with the new EF-S 18-200mm f/3.5-5.6 IS lens, equivalent to 28.8-320mm. Of course, there's a body-only version as well. Though the 28-135mm lens doesn't provide the coverage or all-in-one convenience of the 18-200mm lens, I think it's a better lens, and would recommend that kit over the other and perhaps supplementing with the Canon EF-S 55-250mm f4.0-5.6 IS lens; that dual-lens configuration can be cheaper as well.
For better or worse, there aren't a lot of significant design or feature changes from the 40D. At 1.9 pounds, the body has gained a little weight--about an ounce--but retains the same dimensions: 4.2 inches by 5.7 inches by 2.9 inches. It retains the same comfortable grip and sturdy, partly dust- and weather-sealed, body, as well as compatibility with the old battery and vertical grip. I have the same likes and dislikes about the control design and layout as with the 40D. The series of three buttons above the status LCD--metering/white balance, AF/Drive mode, and ISO/flash compensation--are easy to use, but they feel identical. The status display delivers complete information and duplicates it on the rear LCD. Following the lead of competitors, Canon added the capability to change settings from that back information display, using a combination of the joystick and the big Quick Control dial on the back. Overall, it remains a good shooting design that upgraders will have no trouble adapting to and newcomers to the line should pick up pretty easily.
There are a handful of new features, though no movie capture. Aside from the bump to 15 megapixels from the 40D's 10 megapixels, the most apparent addition is Creative Auto, a new semimanual mode with capabilities you can view as an advanced Auto mode or dumbed-down Program mode, depending upon your viewpoint. All functions in CA are automated, with a few exceptions. Notably, it replaces shutter and aperture adjustment options with two sliding scales--Exposure (brighter/darker) and Background (blurred/sharp)--that implicitly adjust shutter speed and aperture. While it's an interesting idea, it seems too much of a newbie feature to put on the 50D. The Rebel series seems far more appropriate. In CA mode you can also can select single, continuous, or self-timer shooting; Picture Style; photo size and quality; and flash mode (auto, on or off).
The Digic 4 chip enables some other new capabilities, including face detection in Live View mode (up to 35 faces), additional settings for the Auto Lighting Optimizer and high-ISO noise reduction (low, medium, and strong), and user-requested variable raw sizes of 7 and 3.8 megapixels. There are also some tweaks to the autofocus system, for example compensation for pulsed versus constant illumination, and support for in-camera lens databases that enable it to perform vignette correction and ensure undegraded illumination across the entire frame. Finally, Canon has improved the dust prevention, with a fluorine coating in front of the low-pass filter to deal with sticky dust.