EWWW IO uses optipng, which was forked from pngcrush.
There are a few cases where it can perform better still than optipng, but to do so, it requires that you pass very specific parameters to get any of those results. It doesn’t do the sort of automatic optimization that most other png optimizers are capable of.
There are some very good comparisons of png utils at http://css-ig.net/png-tools-overview.html
As for a direct comparison: using optipng + pngout (or in the case of the EWWW IO Cloud, optipng + pngoptimizer + advpng), EWWW wipes the floor with pngcrush.
