[J O L L Y]

Honestly, I think the idea is great, but I'm very skeptic.
One the main problems I see with this is the size ; Most modern phones are very carefully designed to put as much as they can in a (mostly) tiny case.Modular means that the phone will be bigger and possibly heavier than other phones, just like desktop PC's and laptops.Also being this modular means that it will be easier to access and hack a phone, that's the reason Apple makes their products closed and difficult to modify.
Now if Google can show that they can make this work then I'll consider looking into this.