A rich snippet example from Schema.org http://schema.org/AggregateRating:
<html>
<div itemscope itemtype="http://schema.org/Product">
<img itemprop="image" src="dell-30in-lcd.jpg" />
<span itemprop="name">Dell UltraSharp 30" LCD Monitor</span>
<div itemprop="aggregateRating"
itemscope itemtype="http://schema.org/AggregateRating">
<span itemprop="ratingValue">87</span>
out of <span itemprop="bestRating">100</span>
based on <span itemprop="ratingCount">24</span> user ratings
</div>
</div>
</html>
But http://www.google.com/webmasters/tools/richsnippets won’t show a preview.
So, the following words from http://support.google.com/webmasters/bin/answer.py?hl=en&answer=146645 are just lies?
New! schema.org lets you mark up a much wider range of item types on
your pages, using a vocabulary that Google, Microsoft, and Yahoo! can
all understand. Find out more. (Google still supports your existing
rich snippets markup, though.)
It is working absolutely fine.
Google is not obliged to show you preview every time, and here it shows an error when I inserted your give example from schema.org:
I have done it in my website’s news pieces and it shows fine.