The animation is highly distracting. It draws the attention to the button instead of to the menu the user now wants to use. The animation creates a “wow” effect on first use, but that’s exactly the kind of visual affordance you don’t want on a UI element for routine use.
IMO, it works because the button moves much less than the menu itself, which is a large block of color that slides out. My eye is drawn to the larger visual change.
I think it's perfect. It's hard to miss the giant white sidebar that pops up. Actually, I think it's safe to say nobody is going to miss it. Keeping immediate attention on the hamburger icon that now switched to an 'x' gives users information on how to close the sidebar if they wish (as opposed to swiping it left or something). If the "distracting" animation we're omitted, the site could have a different problem that users didn't see the 'x' and now don't know how to close the sidebar. I think this site found a perfect balance on usability.
