There are many services where users author "Reviews."  A few include Yelp and Amazon and further examples are below. 

A review can be represented by:

• <activity:verb> = Post
• <activity:object-type> = Review
• <thr:in-reply-to> = Item reviewed 

The Review object-type was included in a June 2009 draft of the Activity Streams base schema, but it is not included in a January 2010 draft.

Proposed Language

The "review" object type represents a user's review of an object.

The object type URL for the "review" object type is http://activitystrea.ms/schema/1.0/review.

A review object MUST include a thr:in-reply-to element that identifies the object that is being reviewed.

A review MUST include the title of the object being reviewed as either <entry><title> or <entry><activity:object><title>

Examples in the wild

• Amazon
• Apple.com
• Digg
• Hulu implements <activity:verb>... post and <activity:object-type>... review in RSS 2.0 (from this feed linked from the profile):

  
  <item>
        <title> Rob Dolin 
                wrote a review about Community: Pilot.
              </title>
        <description>
          <![CDATA[<a href="http://www.hulu.com/watch/96407/community-pilot"><img src="http://thumbnails.hulu.com/687/50002687/111819_145x80_generated.jpg" border="0" alt="Community: Pilot (season 1, episode 1)" width="72" height="40" title="" /></a> Rob Dolin 
                wrote a review about <a href="http://www.hulu.com/watch/96407/community-pilot">Community: Pilot</a>.
              ]]>
        </description>
        <category>Review</category>
        <guid isPermaLink="false">tag:hulu.com,2009-09-19,activity-11029182-Review-226209</guid>
        <link>http://www.hulu.com/watch/96407/community-pilot</link>
        <pubDate>Sat, 19 Sep 2009 00:20:32 -0000</pubDate>
        <hulu:show-name>Community</hulu:show-name>
        <media:title>Community</media:title>
        <hulu:video-title>Pilot</hulu:video-title>
        <media:thumbnail url="http://thumbnails.hulu.com/687/50002687/111819_145x80_generated.jpg"/>
        <activity:verb>http://activitystrea.ms/schema/1.0/post</activity:verb>
        <activity:object-type>http://activitystrea.ms/schema/1.0/review</activity:object-type>
        <thr:in-reply-to type="text/html" href="http://www.hulu.com/watch/96407/community-pilot" ref="http://www.hulu.com/watch/96407/community-pilot"/>
   </item>  
  

• Qype
• Rotten Tomatoes
• Yelp

Questions

• How to handle rating scales? 1-5 scales vs 1 out of 100... etc
• Different rating types -- vote up/vote down versus star rating versus percentage versus tomatoes, etc.