C#/Mono with Unity, or just plain Mono/.NET. Unlike this it's actually cross platform (ie. you can use it on iOS, Android, all desktops, browsers via plugin, and the consoles), it's been used/tested by many companies, it comes with development tools, etc.
If I want a HTML5 game I'll write JavaScript or CoffeeScript, it's going to have wildly different performance characteristics over other platforms anyway. Java sucks for game development, eg. no operator overloading or value types, it's insanely verbose, and this uses GWT to compile down to JS anyway so it's not going to get any performance benefits from being written in Java over JS.
Avoiding a browser plugin is precisely the reason anyone would ever go with something like this.
As for writing plain old Javascript or CoffeeScript, I'd be all for it in principle, but the game libraries are still a bunch of fairly weak offerings, at least when it comes to general-purpose game middleware. I'm not sure if PlayN is much better feature-wise, but if you're planning to target Android anyways (and hence get your hands dirty with a bunch of Java code), I can definitely see value doing it through PlayN to get a browser version "for free". Doubly so if you've already got a lot of experience and tooling set up for doing games in Java (probably for Android).
Also, PlayN is not exactly unused/untested: Angry Birds for Chrome uses it (http://chrome.angrybirds.com/), and it seems very solid. It's a huge testament to the quality of GWT's compiler that they pretty much cut and pasted JBox2d into PlayN (IIRC they only had to alter a couple of threading-related classes when they brought it in) and it performed on par with the Javascript ports, despite the fact that the port was written exclusively with desktop Java in mind.
If I want a HTML5 game I'll write JavaScript or CoffeeScript, it's going to have wildly different performance characteristics over other platforms anyway. Java sucks for game development, eg. no operator overloading or value types, it's insanely verbose, and this uses GWT to compile down to JS anyway so it's not going to get any performance benefits from being written in Java over JS.