I CANT STAND sites that do that. At a minimum it's frustrating because I tend to hit it accidentally. Then there's the infuriating fact that it messes with the system back/forward gestures on iOS. Of course every website does something different so it's not really something you can actually learn to use.
Worst of all I was on the website the other day where I couldn't use pinch to zoom without it moving me to a different page. It was literally impossible to zoom into the content.
I can't stand all these things where websites seem to try to invente the features that are already built into mobile phones. "Will make a bar that makes it easy to share this!" I can ALREADY share things easily on my phone. And it doesn't waste 10% of my screen at all times. Also, I can configure at the way I want.
Google's attitude that it's "your page" despite the URL and header is complete BS. They clearly hijack it for their own benefit.