"Usable" as a term is a bit of a wash in the browser world. Some people will argue Firefox or Chrome are unusable due to some minor annoyance and others will say filling is still usable. Trying to give an answer though I'd say most will say it works with most sites in a few years time but most would also not recommend it due to security concerns. It's not that they haven't thought about security, sometimes they even try newer more segmented approaches than current major browsers use, just that everything has been done from scratch and the work to make the code safe and battle tested as other browsers would be more than the work to make the browser to that point.

My gut feeling with how far they've come in so little time says it's definitely in the double digit percentage points.