I had a 50D at home tonight — it goes back tomorrow, so there's no time to test everything. I decided to screw around and take a bunch of shots at ISO 6400. It actualy goes to ISO 12800, but those were useless. I was thinking maybe the 50D's ISO 6400 would look like my 40D's ISO 3200.
Well, not so much. I didn't see anything amazing. ISO 6400 isn't bad, though. The 40D's ISO 3200 edges it out, but not by much.
The 50D is nice, but not really worth an upgrade from the 40D. It has a very nice screen, good enough to actually determine if a shot is in focus or not. It has some new bells and whistles. Oh, and 15 megapixels. What that did for me was slow me down with huge files.
It feels and handles just like the 40D. One of my beloved user setup slots on the dial has been replaced with a new "Creative Auto" mode. Ugh. And the dial is a highly reflective aluminum color that I can see burning out someone's cornea on a sunny day.
I'd say go straight for the 50D if you're making the leap from 20D or 30D, but if you've got a 40D, keep it. Save up for the 5D markII or wait a year or two until the 60D comes out. The 50D is a wonderful camera, no doubt, but other than its great screen, there's nothing there tempting me to upgrade.
Here's one at ISO 6400 that came out better than most of the shots I snapped.