Image compression and device-sensitive transcoding are important for mobile and emerging markets, and are somewhat challenging to deploy on your own. For larger sites we acknowledge there is some tuning & thought that must be applied, and we'd be happy to help via the discussion groups (mod-pagespeed-discuss, ngx-pagespeed-discuss). We are also working on a new release that will make this easier. For smaller sites, things should work out of the box. As with all PageSpeed issues, please let us know if they don't, or you think there's something specific to improve.
We've heard user concerns about SEO, and are happy to report that this should be improved with our latest beta release (1.0), which adds 'link canonical' attributes to image response headers. This allows search engines to point back to the origin image.
Please give us a shout on our discussion group or bug-list and we'll try to help sort through any others issues that you see: https://github.com/pagespeed/ngx_pagespeed/issues?q=is%3Aope...