I've got a Nissin Di866 and it's fantastic. I use it mostly on my 60D while I use my 580EX II on my 5D3 but the Nissin has been used off camera quite a few times to great effect. I got a Nissin Di622 MkI for the wife when she had a 550D, and she hadn't ever used it on the 550D before she got a 650D and the Di622 MkI isn't compatible with the 650D so I've just sold it. I'm thinking of letting the mrs have my Di866 and getting the MkII or a different Canon flash, maybe a 430EX II this time.
The Nissin in the classifieds in the post above is no longer for sale as I've bought it. I was so impressed with the original Di866 I got that I've handed it over to the wife and will now use the 'new' MkII on my 60D instead. Had it not been part of a package when I got a 5D2, 24-105 f4 L and 580EX II then I wouldn't have gone for the 580, but would have gone for another Di866 instead.