I am (work at Lucid Software: www.lucidchart.com www.lucidpress.com). We're using in conjunction with Closure Library, Clutz, Tsickle and Angular AOT template compilation to get great code sizes.

And Google has always been a heavy user of course.

Closure Compiler is preternaturally good at dead code removal, inlining, minification. But you have to use advanced optimization mode, and write JS amenable to its static analysis. If you're willing to put in that work, you'll get fantastic minification. If you're not, you should just find another tool.

Scala.js includes Closure Compiler adv optimizations right in the standard workflow, so it's a good bet any Scala.js project will be using it.

May not be the angle your asking for, but clojurescript relies on the Closure compiler.