I do have a 5D Mk II myself and I used the lens in some situations. To reply to your question, it does work with the 5D MkII, the only thing you cannot zoom all the way out because you will see the edge of the peddle flare guard. In few words zooming till 15mm doesn’t create any problems and you can fully used the lens, if you want i can send you few pics actually i attached the link to my flickr page.
I refrained from getting the Canon fisheye as i could read much better reviews about the Tokina