The main benefit of Screenleap over a Google Hangout is that the people you are sharing your screen with don't have to install any software in order to view your screen. All they need is a web browser.

The average Internet user spends 2.5 hours per day on email and most Screenleap users send links as their primary means of notifying viewers. We believe the Gmail browser extension makes it even more convenient to share your screen because we can auto-populate your emails or chat conversations with the share link so you don't have to copy-and-paste it yourself.

One thing that I don't like about Google Hangout's screen sharing implementation is that I can't seem to view other participants screen in fullscreen. I often find myself squinting at the screen trying to read something.

If Screenleap solves this then I'd be willing to give it a try.