Just got mine a couple weeks ago to work alongside my 1D MKIII beast! Images are similar up until ISO3200 but then the 5D takes over for sure.
Now that I FINALLY have a full frame camera after 15 years of photography, I'm looking to replace the stellar Samyang 14/2.8 (manual focus though) and 24-70/2.8L lenses with something that has a wide to medium range and maybe IS for video.
The Samyang is so good, but I want full autofocus combined into one lens. The 24-70 copy I have is also very good but I have a feeling the 5D MKIV out resolves it by a mile.