To activate Google Maps in WebGL, open Google Maps and check an option to "try something new" on the left bottom of the screen. I only noticed this note when I opened Maps with Chrome, but Firefox should prompt you as well, according to Google (for some reason, I could not get Firefox to run the Maps WebGL version). If you are using IE, you won't be able to try WebGL Maps, because there is no support for WebGL in IE.
Once activated, you can see WebGL in action in the 45-degree angled view, for example when you view the Colosseum in Rome, Italy. There is a new feature to rotate the view, which can be accessed right above the Streetview figure. Rotating is still limited to four views, but the animation now makes changing views appear much smoother. You can also enable Streetview, which is now support natively and not via plug-in. The rotate feature also works in Streetview.
Other than that, there is no other benefit; I could not quite confirm Google's claim of higher image quality. If you are taking MapsGL out for a spin, be sure to view Google's guided tour, which provides a quick overview of the features and how they can be controlled. What struck me, was that, in direct comparison with IE and the non-WebGL version of Maps, there is really not that much of a difference. Sure, the adjusted 45-degree view isn't animated, but IE9 is, at least on my PC, the far better browser to use maps. Dragging and zooming maps is much smoother in Microsoft's browser than in the WebGL version of Maps in Chrome.
MapsGL, as Google calls, is nice to have, but it's not a must have that will scare Microsoft enough to convince the company to build WebGL into IE.