I use it all the time to make minified versions of my plugins. It is my favorite online compiler and it seems to produce the smallest code. The only time I ever had it fail was when I was trying to minify the original tablesorter code which uses `eval()` to make multi-column sorting work. GCC would strip out all of that code because the evaled variable wasn’t defined or used until the code was evaled. So I had to use Uglify instead.
jQuery uses [Uglify.js](http://marijnhaverbeke.nl/uglifyjs), but I remember them saying they were working with Google to make GCC work for builds. Uglify is popular because you can use it with node.js.
There is also [YUI compressor](http://refresh-sf.com/yui/), which I’ve only used once or twice, so I don’t have any constructive feedback to give about it.
And lastly, of the compilers that I know about, is [packer](http://dean.edwards.name/packer/). It’s the oldest, and sometimes produces the smallest compiled results because of symbol obfuscation.
