Though it's already been superseded by the just-announced 450D/Digital Rebel XSi, the 400D or Digital Rebel XTi is a great choice for beginning photographers hoping for the flexibility of a digital single-lens reflex camera (DSLR)
The main points of getting a DSLR over a cheaper compact digital camera are speed, manual control, and low-light quality, and the 400D doesn't disappoint considering its low price. The 3FPS continuous shooting in JPEG mode may seem rather mediocre on paper, but it performs just fine in the real world, even with track and field events. Manual control isn't the best with only one control wheel, but the 400D's meter is accurate enough for shooting in aperture-priority mode most of the time so this isn't a problem. ISO is also quickly accessible through a dedicated button.
The most visible improvement over its predecessor, the 350D, is a much larger 2.5" viewing screen. While this necessitated the removal of the second status LCD, the new shooting setting display mode works fine in conjunction with the face detecting sensor which shuts off the LCD to save battery when your face is up to the camera.
While the 400D is great for the beginning photographer, it does have its drawbacks. More advanced users will feel limited by the included kit lens' (Canon EF-s 18-55 3.5-5.6 II) slow maximum aperture and mediocre optical quality, and the 40D prosumer camera offers a much more robust build quality and ease of control.
Considering its target market, the 400D is an excellent camera, and well worth considering over the newer 450D. It may not last a developing photographer forever, but it's a great start into the world of digital photography.