I shoot almost exclusively indoor action sports (competitive cheerleading). Even with fast lenses, sometimes the lighting is so poor that shutter speeds suffer. I just shot a competition this last weekend with the new 50D in a convention center WITHOUT any stage lighting, and went to ISO6400 just to see what happened. WOW!!! Shutter speeds back into the 1/500 to 1/640 range and quite acceptable noise levels. This is the first camera I have felt comfortable shooting in JPG. The results were great, and while I probably wouldn't want to blow up prints to posters at that high an ISO, at 8x10 or full screen web shots, they are perfect.
With all the complaining I heard about the AF sensors, it sure does a great job for me. MUCH more accurate than the 30D, and for the FIRST time, AI SERVO is usable for low light applications. I don't even want to TRY the 1D now, if its that much better......I can't AFFORD to upgrade again so soon. :-)
All in all, a worthy upgrade from my 30D (which is now my new backup).