First, let me start off with that these are purely my opinions, and should be used for what they are worth. As background, I bought a 50D a little over a month ago. I currently have a 10D, 20D, 5D, ID mk IIn's (x2), and a Nikon D3, so that is what I am basing my comments on.
For the first time buyer or someone moving up from the XXXd models
This is a perfectly capable camera. It does just about everything well. Shutter lag is almost non-existent, focus is very fast, and it pretty much does everything you want it to do. ISO performance in real world use if very acceptable up to ISO 1600. Above that it becomes very tricky. It will deal with noise, but you had better nail the exposure otherwise it is utterly unimpressive. Viewfinder is acceptably bright and usable. Size and weight are just about right. Given the competition between it and the D300 from Nikon and the recent price reductions with the Nikon, I would let lens choice be your driver here. The D300 (yes I have shot it) is a slightly better camera, but each is about equal when you get to the point of putting pixels on screen or in print. The Nikon shares more stuff with its big brother, but it comes at a cost. Not huge, but it is there. But the Canon is a very good camera for 90% of the people out there. It covers all the bases pretty well and I would have no issues recommending it. The one cautionary part is if you want to use 15 mpx, make sure your lens is up to it. In the past, the difference between my Sigma 70-200 and Canon sister lens was pretty much unnoticable. But at 15 mpx, the canon is starting to show off a bit more. Lenses are starting to show their weeknesses at that resoution. In the final print, it isn't noticable, but on screen during editing, you see it.
Canon Upgraders
Here the story changes dramatically. I just finished reviewing my shots from the last month getting them ready for storage, and one thing that hit me squarely in the face. The 50D does not enable me to do anything I couldn't do with my older 20D. Sure the review screen is better, but I wouldn't rely on it for focus check. The menus are different - not better or worse. Either are easy to navigate once you have learned them. The 50D does everything a little better. Focus slows down less in low light, but it still slows down. Shutter lag is gone. All the other problems still exist. I still need to pop the battery when using non-Canon brand batteries from time to time. It is nice that it shares many of the accesories of the other Canons, but it isn't a system like the Nikon family is. I have found some older accesories also will not play nice with the 50D. Both my Sigma 1.4 tx and 2 x TX loose autofocus even when used with my fast lenses. My Canon 28 f1.8 won't work at all. I hate that lens, so no big deal to me though. Don't get me wrong, the camera feels good. But if you have a 20/30/40D, I would NOT recommend upgraded to the 50D. Again, in true Canon style, there are improvements, but the are small and in ways that don't help the image. If anything, I am still able to squeze out more workable images from my 20D than I can files from the 50D when shooting at ISO 1600 in action based shooting. The files seem more foregiving. Studio shots and the like where you can make sure you have the exposure dialed in, the 50D does produce very nice images all the way up through the to H2. If that is what you do, it will out perform the older cameras. But you have to have the exposure dialed in perfectly. If you have a xxD based camera, and looking to upgrade, go to the 5D, thats where you will see true differences.
Just my two cents worth.
