most people will agree and many will disagree with me but the 50d was not a great upgrade from the 40d. simple as that. and since most of the forum has 40ds... they (and myself) are justifying the purchase of their 40d i guess. Honestly I dont see the 50d as a bad camera, just not a worthwhile upgrade from the 40d.
And the 40D wasn't a big upgrade over the 30D, and the 30D was not a big upgrade over the 20D, etc.
The story is the same year after year. The standing rule for the past 3 years from what I have seen is that only sensible thing for most folks is to skip every other release. There is no real definition of what the ideal upgrade of one body over the prior is, everyone has their own subjective opinion.
Bottom line, buy what you want, need and/or can afford, and own that decision. If you don't like it after the fact, in most cases it is nobody else's fault, but your own for having purchased it (unless you want to discuss the AF issues on the MKIII when it first came out).
If you are happy with what you have, then keep it and use it.
There are only 2 decisions really, and neither of these will likely have anything to do with Canon.