I immediately tried it against Spry's largest file, SpyData.js, and here were my results:
SpryData.js uncompressed: 126k
SpryData.js minified: 66k
Woah! Almost a 50% reduction in size.
I then tried it against the Dojo ShrinkSafe version of SpyData.js that I had created last month:
SpryData.js compressed with ShrinkSafe: 72k
SpryData.js minified: 64k
Wow, even more reduction if I run through the file through ShrinkSafe first!
I did try this on the Ext-All.js file and also saw a reduction of 30k in an already compressed file!
This is really a good utility to have and the results are outstanding. I would say to use this in conjunction with another packer and some http compression for really boosting performance on your web apps.