It's about a third. Imagine if your income went up by 30% in 24 hours, I reckon you'd be pretty happy about that.
Also - it tends to take months for a new version of iOS to reach a large number of users, and years to reach everyone. So a rapid growth rate (probably not 30%, but still fast) is likely to be sustained over quite a while.
Yeah, I'm sure a lot of people are trying it out. I'm curious what the other options look like and how many continue using them after the initial install.