I've tried to jot some thoughts down but it has ended up being a bit rambling and perhaps a bit self-contradictory as my thoughts have developed while writing. Anyway, here goes....
I am happy with the 50D, but the context for that happiness is that I bought it as an upgrade to my 30D. Up till now I have actively used my 40D and 30D side by side, typically with a long lens on one (70-200 f/2.8 IS) and a wide lens on the other (17-55 f/2.8 IS), or a very long, slow zoom (100-400) and a less long fast zoom (70-200). For weddings it's a real advantage to be ready to shoot anything with a focal length range covering 17mm to 200mm all at f/2.8, without faffing about. For vacation/travel it's great to be able to shot landscapes and yet be ready to pick off some interesting wildlife without faffing around swapping lenses and completely altering camera settings to suit the different needs of the subject/scene/glass.
I've always been happy with the IQ from my 30D but, compared to the 40D, the handling and operational performance is less good. Specifically, the button layout is different between the 30D and 40D on the top controls, so if you want to quickly adjust ISO, or metering pattern, without removing your eye from the viewfinder, you have to remember which camera you have at that moment and where the right buttons are. That is just a pain when the action is busy. Then the other big benefits of the 40D over the 30D are the bigger brighter viewfinder, permanent ISO display, faster AF, custom exposure/configuration settings, Live View, personal user settings menu and no doubt a few other things. To be honest I don't really care that the 40D has 10MP so long as there is no harm to IQ. Basically I am entirely happy with my 40D.
Now, moving forward to today and the 50D. Well the first thing to say is that it is everything that the 40D is and perhaps a little bit more. As a replacement for my 30D, and to work in concert with my 40D, it is a good upgrade. Benefit number 1 for me is the new microfocus adjustment. I have 9 lenses and, although the adjustments are generally small, I think I have adjusted at least half of them. The biggest benefit is with my 100-400, which seems to have become noticeably sharper, even with an adjustment of just +2 (out of 20) dialed in. Several people have said that the AF is quicker with the 50D, despite the fact that it is the same AF technology, but maybe the DIGIC 4 processor is giving it the edge for high speed AI Servo action shots. It's hard to make a direct comparison without controlled tests but I have to say that in the field I find the 50D AF to be excellent, locking in quickly and doing a great job of tracking, so long as I do my part and keep the AF point where it needs to be. I don't know whether the 40D is just as good but I have pushed my 50D harder, shooting action in far lower light than with the 40D, and got very satisfying results.
My original gut feel was that I was not enamoured with the 15MP idea, although maybe that's just something I will come to accept more readily as time goes on, as I do make an arguement for it further down this post. It is, or so I thought, a triumph of marketing over common sense. I would have much preferred Canon to bring the latest gains in sensor technology and image processing to another 10MP sensor. 15MP is too much, I think, on these small sensors. Don't even get me started on compact cameras with 14-15MP. It's a crazy world. The other things about 15MP is, as someone else mentioned, the sheer bloody size of the raw files. I can't say I'm thrilled at all at routinely having to handle files around the 20MB mark. The 40D was bad enough at ~14MB. I have 1TB storage on my main PC, so I'll be OK for a while, but I just had to upgrade my laptop drive from 120GB 5,400rpm to 320GB 7,200 rpm just in order to keep pace. It was only £65 to do that and well worth the expense, but nonetheless....
Now, what about IQ? Well, for personal use I view my images on a 40" 1920x1080 LCD TV. I only need ~2MP of data to fill that screen, so what on earth do I need 15MP for? Well, the truth is that I don't need 15MP. Sure, I can crop away fairly aggressively if my glass is not long enough to reach my subject and fill the frame, and for that purpose I think there are some benefits. It's certainly cheaper to buy a 50D and a 400mm f/5.6 lens, and then crop a shedload of pixels away, than it would be to buy a 1D3 and 800mm f/5.6 and have no need, or opportunity, to crop. Of course, for sheer IQ the 1D3 and expensive glass would deliver better results, but it would weigh a fair bit and cost £10,000 vs ~£1,600.
With the 40D I liked to try to crop in a 16:9 ratio, aesthetics permitting, at a pixel size of exactly 3840x2160, allowing me to output my final JPEG resized to exactly 50% to create a wonderfully sharp image for my display. With the 50D I just have so many more pixels that I don't really need, unless my glass is too short, or my composition is way off for some reason - perhaps tracking action with the centre AF point and needing to crop to improve the composition. There, the 50D may give me an advantage. But, for regular photography, where my glass is long enough to fill the frame and I have the time to compose accurately I simply don't need those giant files. This is where the new sRaw1 format is of real interest. It gives me a decent number of pixels, without waste, still allowing me to downsize quite a bit and sharpen things up nicely. File size is reduced and, at high ISOs, some of the inherent noise is mellowed quite well. With the 40D the sRaw format had very little use, the files simply being way too small. With the 50D, sRaw1 is not much different in pixel size to the 30D native raw format.
I do also shoot weddings, as second shooter, and I hand my processed files over to the main tog for printing. So long as I can deliver well composed, well timed, well exposed, clean, sharp images that are at least 3,000 pixels on the longest side then he's good to print up to 10x8 with no problem and (especially if I supply more pixels) considerably larger prints if need be. So the 4752x3168 full raw files from the 50D should give loads of options, and the 3267x2178 sRaw1 files should be more than adequate too.
When you get to comparing things at a pixel level, the 50D will struggle to match the noise levels from the 30D and 40D, but people don't shoot to capture pixels; they shoot to capture images. I think we have reached the stage in digital photography where it is pretty meaningless to compare individual pixels, especially between sensors of such vastly differing pixel densities. The high density sensors are likely to exhibit equal or perhaps even a fraction more noise. Any defects in lens sharpness, focus accuracy, camera shake, subject movement will all be more readily revealed at the pixel level. That is no fault of the camera. That is the fault of everything except the camera. So forget the pixels. Look at the overall image. Can you get a *picture* with the 50D that is as good or better than the 40D or 30D? The answer has to be "Yes.". Does the 50D give you more options to use higher ISOs than the 30D and 40D? Yes. Can you shoot at very high ISOs and control noise effectively? At 6400 ISO and using sRaw1 the answer is - Yes. Can you shoot at 12,800 ISO and deliver a useable picture - No question.
I think I've rambled long enough, so to summarise - The 30D is a very good camera. The 40D is a better camera and fantastic value at today's prices, especially with the Canon rebates available through to January. The 50D is, in simple terms, the best camera out of all of them, offering more options to get you the image you want. Forgetting about pixel peeping, with microfocus adjustment your lenses will never be sharper. The AF is fabulous. There are real options to juggle pixels vs ISOs to deliver the image you need under difficult conditions. 50D prices started out at absurd figures and it was impossible, IMHO, to justify spending £1200 on that camera, with the 40D selling at around £530 after cash back. I paid £795 for mine. That's still a big premium over the 40D. If you can use the flexibility that the new ISO ranges, sRaw formats and sheer number of pixels offers then go for it. If your photographic needs are less varied and less demanding then save your money and get a 40D while you still can. You will not be disappointed.
Final word - If I were to take just one body with me when I go shooting, which would it be? - the 50D.