I bought a 50D with 18-200IS to take on a trip to NY. I've been testing this body a lot since I got back with my better lenses and maybe I'm rushing to judgement here but I wonder if Canon hasn't tried to pack too many photosites onto an APS-C size sensor essentially turning it into a big point and shoot.
Using standard metering, at ISO400 I'm getting a lot of noise which the RAW convertor has to deal with at the expense of sharpness. This is to the extent that 100% crops of the same scene look sharper and cleaner from my 1DmkIII then the 50D files viewed at the same reduced size. This kind of wastes the extra resolution/cropability of the 15mp sensor.
Canon bodies have always retained great sharpness at high ISO but this sensor seems initially like a big step backward. The 1D3 has always afforded me great exposure latitude but with the 50D it looks like I will be back to exposing with the histogram to the far right even at normally low ISOs which in turn means extra processing in the convertor.
Viewing shots at normal web size the 50D images look OK but certainly no better then the 1D3.
If things don't improve I may sell this body and get a 40D. I like the smaller body along with the 18-200 as a travel setup and the general consensus is the the 40D is a great body from an IQ standpoint. Certainly everything I've seen from it has been very good.
One other caveat with the 50D: there was a huge difference in IQ between the 18-200 and my L lenses, it appears the 50D needs good glass to even come close to realizing it's IQ potential.