Recently, I moved from a 50d to a 5d and, so far, I am very happy.
For me, the most significant advantages of the 5d over the 50d are:
- Less noise (the 50d was pretty noisy)
- Shallow DOF
- Better image sharpness
- Better/more consistent metering
- My EF lenses can now be used at their "real" focal lengths
The most significant disadvantages are:
- Lack of live view (albeit, in most situations you find a way around it if you don't have it)
- More size and weight
- Lack of AFMA (albeit, I seem to be lucky: my lenses so far all focus spot-on)
- The AF points are spread wider on the 50d, which I sometimes miss on the 5d
All in all, I prefer the 5d over the 50d. IMO, it delivers significantly better image quality, which is most important for me. In addition, I kinda like the 5d, because it feels so "pure" as a photography tool and it is a great joy to open the RAW files on your computer screen and see the quality it produced in the field. In contrast, the 50d output in difficult conditions often disappointed me and required much postprocessing to reach acceptable quality.