From his August 20th article talking to Kirk Gray of APUS: "The more I read about Gmail tabs, the more I think a bunch of folks are getting worked up over a whole lot of nothing."
From his August 27th article quoting another author's extreme concerns: "You know what I don’t agonize over? Gmail tabs. Send stuff people want and whatever the various inbox providers do with their interfaces will have little effect on you."
He's right. So far, the people I'm talking to aren't really seeing any sort of significant dip in stats. Time will tell, maybe it's just too new. But right now, I'm not worried.
There's a good caveat in Ken's second quote. It's worth calling out: "Send stuff people want." If you're shoveling large amounts of low-value content at recipients, Gmail tabs makes it easier for those recipients to ignore you. If so, you're probably going to have some struggles.