Hi Peter, I had a Di622 MkI which was excellent when it was used with a 550D and a 7D. It did start having compatibility issues when I got a 650D and a 60D though but generally it was a fantastic performer. The Di622 MkII is compatible with all the newest bodies though so you wont have any problems with it. For the price they are an excellent flash, really well made, fast cycling and very powerful. A decent set of batteries seems to last longer in the Nissin compared to Canon guns as well, I got 800+ shots from a set whereas the Canon goes after about 600 shots.
I have a Nissin Di866 MkII and a Canon 580EX II and TBH there's not much between them in features. I usually use the 580 on my 5D3 and the 866 MkII on my 7D, the Canon does seem better on E-TTL settings, although when the Di866 is on auto setting it acts more like E-TTL anyway. I also use the Nissin as off camera flash set off by the Canon and that can also be set up on the Di622 as well, which is very handy for creative purposes.