I've used WebGL to visualize scientific data. It's nothing special, but the size of the data was large enough that anything like SVG or even Canvas were just slow. There is a lot of interesting stuff you can do even just in 2D in WebGL, because you're closer to the hardware.

What's annoying is that you don't get double precision or any of the compute stuff on WebGL currently. The interesting stuff isn't just graphics, but also doing computations directly in the browser.