How to grab a user's Fivestar voting value with Rules?
I finally found a solution for my problem. This is my exported rule:

{ "rules_new_vote" : {
    "LABEL" : New vote",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "php", "radioactivity", "comment" ],
    "ON" : { "comment_insert--comment_node_article" : { "bundle" : "comment_node_article" } },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "comment:node" ], "field" : "field_popularity" } }
    ],
    "DO" : [
      { "variable_add" : {
          "USING" : {
            "type" : "integer",
            "value" : {
              "select" : "comment:cid",
              "php" : { "code" : "$comment = comment_load($value);\r\n$vote = $comment-\u003Efield_stars[\u0027und\u0027][0][\u0027rating\u0027\r\nif ($vote == 20) {return -4;}\ r\nif ($vote == 40) {return 2;}\r\nif ($vote == 60) {return 3;}\r\nif ($vote == 80) {return 4;}\r\nif ($vote == 100) {return 5;}" }
            }
          },
          "PROVIDE" : { "variable_added" : { "rating" : "Rating" } }
        }
      },
      { "radioactivity_emit" : { "data" : [ "comment:node:field-popularity" ], "value" : [ "rating" ] } }
    ]
  }
}


The trick was to store the cid as a new variable and to use that value in the PHP evaluation (the proposed values in my code can be changed to your needs). I've no idea why this wasn't possible in an easier way via the UI, but maybe it's a bug in the Firestar module.