It doesn't entirely disappear; I agree with MR that this feature is kind of neat and almost seems like it should have always been there. Everything is still there once you tap the URL, which if you're navigating to a new one you'd have to do anyway (well, everything except a one-tap way to see tabs -- I turned more back on), and there have always been gestures for things like Back without needing the navigation bar, though it's there if you want it. So, it leaves a bit more room for content on small screens. (Not exactly groundbreaking, but...I don't hate it like I thought I might. Still wish tabs weren't two taps or mysterious swipes away without me turning more back on, though.)