This is amazing and feels very fluid. I think it is part of a web-based software trend that will surprise a lot of people who think of the web at best as an interactive document display system. Another app that has a very snappy UI like this is http://OnShape.com CAD software, which is full featured 3D solid modeling without any plugins (all JS and WebGL).
The difference is the browser vendors are in control of the implementation that will make it slightly more secure, but it's still a long way from the HTML/JS/CSS environment people though would replace plugins.