I've just joined the club after buying one from a member here.
I was initially a little disappointed with the image quality. I was expecting a significant leap over my 20D particularly in noise and dynamic range but so far I've been a little underwhelmed to say the least, I expect my expectations were too high. I was quite disappointed in the amount of 5D noise although I have found that high ISO 5D shots whilst initially looking just as noisy as the 20D's respond better to noise reduction.
I bought a Siggy 85mm f1.4 to use on the 5D, cracking lens and better than their 50mm f1.4 I think. One good thing is that the shutter is significantly quieter than the 20D's, which is a total disgrace IMVHO. One bad thing is that the 5D+85mm is significantly heavier than the 20D+50mm and of course, very annoyingly, there's no inbuilt flash. Why Canon doesn't fit one to its full frame jobbies when every other SLR/DSLR I've had to date had one I just don't know.