Chrome Apps were in no way replacement for XUL. They could not change the behavior of the Chrome itself, they just opened their own window and did their thing there. Not really an extension, more of a separate app.
To talk on more specific terms, which XUL extension did you replace with Chrome App?
I finally ended up using alternate tools since even the replacement Chrome Apps were pale imitations of what Firefox offered. The good ones needed to use Nacl just to do basic stuff and still couldn't reach what XUL extensions natively supported.
But I changed my browser to Chrome since Firefox lost its value proposition. It had a differentiating factor of superior experience and features that it chose to deliberately abandon.
To talk on more specific terms, which XUL extension did you replace with Chrome App?