So far, from an existing customer's POV, I'm seeing a lot of downsides, for no compensating upsides.
1) Dealing with Tor, who didn't bother reply to my email when I was trying to find out where an international customer could actually buy their ebooks. Admittedly they've made their books a lot more available to international customers since then, but I've still got a sour taste in my mouth.
2) New book size which doesn't match the previous ones, or even fit on the same shelf.
3) The possibility of losing the digital portion of my subscription, for no change in price.
OK, the first point is purely personal, and is regarding another company's poor service which isn't Paizo's fault, the second is also more a personal peeve rather than anything actually damaging to me, but losing the ebook from the physical subscription would be a real cut to service for the same money.
I dunno whether I'd cancel over it from the information we've been given at this point, but I have to admit I wouldn't be a happy customer either.