One major issue you should be aware of is that the 5D has a design flaw that may cause the mirror to be detached from its housing. The fix is fast and free but it's still a hairy moment when the camera stops working and you hear something is rattling inside. Also, damage to the rear element of the lens is theoretically possible but not likely. I don't know if the issue was fixed from the factory at a later stage but I have a '06 model that it happen to.I had the mirror come loose on my 5D classic whilst the 851.2L was attached . At first I could not detach the lens, I was afraid the back element would be badly scratched by the loose mirror, fortunately it was not and Canon replaced the mirror free of charge because it's a recognised fault with the 5D.
With my 5D the exposure is always out in Av mode, mostly results in an over-exposed image, Does anyone else have this problem with Av? I've given up on Av and just use Manual.
The small screen is annoying and does not give an accurate representation of the image, although the advantage is when downloaded to the computer and seen p r o p e r l y one is pleasantly surprised at the images this camera is capable of producing