I guess others have already mentioned this but anyway, Mike Mearls on Twitter has clarified that introducing class groups will not affect the design of classes themselves, i.e. they will not modify classes in order to shoehorn them into specific class groups.
That was my main worry about class groups, thus with that hopefully set for good, I'm not really worried anymore. The matter with magic items is still there, but it's definitely less important to me.
That's a very good thing.