Got around to updating today (didn't want to before, had a couple of gigs imminent). So far, straight YN565 on the camera, and YN622/YN565 mix are behaving as expected. Exposure is good, communication good in terms of changing settings etc. I have some other shots I want to do later this week, if I run into problems with settings or my my 568 flashes, I'll drop a note back in here.
I have a Meike grip and 3rd party batts (Opteka) are behaving the same as before - when inserting new ones, I get the message about irregular communication, and a blinking battery LED (the latter every time I turn on the camera) but after taking the first shot I get accurate battery capacity readings.
@Dave - thanks very much for reporting back. I am in the middle of a product shoot and cannot risk it but will try to upgrade in the next week or so.
Is there a way to save the existing firmware out of the camera to support a future downgrade in the event the upgrade turns my camera 'pear shaped'? Also, does the upgrade wipe the camera settings or are they retained?
Admittedly I have only done an upgrade once with my 40D and it went well so there was no need to revert back to the earlier firmware.