I'd be curious to see how viable this would be in the long run.
Let's assume that Google Domains will reach the "launch publicly" stage. We already have a plethora of excellent, established registrars. Namecheap is constantly doing something great for customers and attracts new folks with creative campaigns. Name.com places a great deal of emphasis on new TLDs (I'm not sure how much business it actually drums up - I have no interest in .NINJA domains), and GoDaddy is... GoDaddy. Then we have others, like Hover, which I have no experience with - but Hover in particular seems to do well. Dynadot's also pretty solid.
So... how would this end up playing out for Google? Just from what I've seen so far, it looks like they're relying on very competitive pricing (with the automatic inclusion of WHOIS privacy) and interface simplicity. They've got flawless Wallet + Google Apps integration as well, but I can't see this being a hugely important "make or break" factor for people. And naturally, they have one big thing going for them: it's a Google venture. The latter doesn't particularly comfort me. I can see them dumping the domains division fairly quickly if things don't pan out. This is fine, but it'll also give us an idea as to how dedicated they are + what they're willing to pour in resource-wise if it doesn't transform into a self-sustaining entity within ['x' amount of time].
I'd be hesitant to transfer my existing domains over, at least for a while, due to this uncertainty of viability. I can't tell if they're viewing this as a long-term investment or simply just testing the waters. But hell, I'm still interested.