How to create a rule to conditionally send a notification email, based on field value? In the Article content type, I have a field called "Assign to" which is a reference to a User entity. So that the users (editors) can know witch articles are assigned to them by the administrators.

How should I configure the Rules module to send an email to editor when article is assigned to an editor, telling editors witch article is assigned to them, and who assigned this article?
Here is a sample of a rule (in Rules export format), which will send an eMail to the userid selected via the field field_article_reviewer (= a field I added to my "Article" content type that is defined as an entity reference to a 'user'), and this whenever content of content type "Article" is saved:

{ "rules_notify_editor_about_outstanding_review" : {
    "LABEL" : "Notify editor about outstanding review",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_presave--article" : { "bundle" : "article" } },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "node" ], "field" : "field_article_reviewer" } }
    ],
    "DO" : [
      { "mail" : {
          "to" : "[node:field-article-reviewer]",
          "subject" : "Article waiting your review",
          "message" : "[node:author] has submitted a new Article post waiting your review.\r\n\r\nYou can review it at: [node:edit-url].",
          "from" : "[site:current-user:mail]",
          "language" : [ "" ]
        }
      }
    ]
  }
}


If (before importing this rule in your environment) in the above exported rule you'd change the field field_article_reviewer to match with your field name of the editor, you should be pretty close to getting it to work.

PS: I'm assuming this question is about D7.