It has a certain "maturity" to it, but as it is basically a framework more than an out of the box CMS, adoption has been greatly limited… Look carefully at the list of known Plone websites here:
Notice that, for the most part, these are large governmental and international NGO sites – the types of sites trying to modernize and integrate with a horde of proprietary legacy systems in the most flexible and stable way.
For a "from scratch" site, it's unlikely that Plone would be worth your time compared to more streamlined/focused solutions, or those that provide more "ground level" support with pre-built modules and integrations (drupal, wordpress, etc).
Simply put, if you aren't measuring your development budget with more than 4 zeroes, and/or desire to have dedicated CMS staffing in-house, it's probably not a cost-efficient solution.
Plone – what are peoples experiences? Looking for unbiased feedback