I just upgraded my 400D to a 50D yesterday. I`ve only taken a few shots so far but my first impressions so far are very good. In particular, over the 400D I`m impressed by.
1: High ISO Performance. This is quite incredible given the megapixel count at 1600 and still very impressive at 3200. Opens up opportunities for low-light photography which simply didn`t exist when using the 400D.
2: Handling, and in particular, the fact that far less pressure is required to take a picture than with the 400D, which reduces shake.
3: Screen. This really is impressive and more than adequate for the purpose for which it is intended.
4: Resolution. I wasn`t really bothered by this when buying the camera, and to be honest was a bit dubious as I was worried about its potential affect on high ISO noise. However, after reviewing some initial shots I`ve been blown away by the detail. Although I still don`t think that I require such resolution, it certainly allows for a tighter high-res crop.
5: Horizon set straight. Believe it or not but I was never able to buy a 400D with a correctly set horizon (always slightly on the **** even when using a spirit level). As expected, this is not an issue with my 50D.
6: Autofocus. So fast, and accurate.
Oh...so far I`ve tested with my Tamron 17-50mm Di II. I`ll do some tests with the Tamron 90mm Macro later......
Hope this is of some use to somebody.