I have a 30D, 40D, 50D and 1D3. I have written in a few forums about my thoughts on the 50D, especially vs the 40D, and you can find my most recent ramblings in this thread....
http://www.birdforum.net/showthread.php?t=135541
More here, including some duplication, but with additional test results....
https://photography-on-the.net/forum/showthread.php?t=665140
In short, the 50D is a better camera than the 40D. Whether for you, personally, the improvements have meaningful value is down to you, your glass, your shooting conditions and what you do with your images. Only you can determine if the extra cost is worth it - for you. To summarise genuine tangible benefits I gain from the 50D, here they are....
- Ability to use a "real" 3200 ISO. I've had to use 3200 ISO for paid work at a wedding.
- Ability to fine tune AF calibration. I have nine Canon lenses, plus a 1.4X teleconverter. That's 11 sets of glass in total. All bar one or two of the 11 benefit from AF microadjustment.
- For small bird shots my glass is just not long enough. I need to crop, hard, and the 50D gives me more pixels remaining on my subject.
- The high resolution screen and high resolution preview image allow me to confirm focus perfection both before and after taking the shot.
- Altogether there are several features which, added together, allow me to go home with sharper, more detailed shots than I could with the 40D.
It might also be worth reading this short commentary, to dispel some myths about the 50D and whether the resolution benefits are realisable....
http://www.bobatkins.com …resolution_confusion.html
In some situations I find the 50D to also be better than the 1D3. You can see my reasoning and further discussion from post #278 onwards in this thread....
http://www.talkphotography.co.uk …thread.php?t=86733&page=7