this.value works

components.my-component.js

import Ember from 'ember';
const { computed } = Ember;

export default class MyComponent extends Ember.Component{
  // model
  constructor() {
    super();
    this.prop = computed('model', function() {
      return this.model.length;
    });
  }
}

controllers.application.js

import Ember from 'ember';

export default class Application extends Ember.Controller {
  constructor() {
    super();
    this.appName = 'Ember Twiddle';
  }
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{input value=appName change=(mut appName)}}
<br>
<br>
{{my-component model=appName}}

templates.components.my-component.hbs

{{yield}}
{{prop}}

twiddle.json

{
  "version": "0.13.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.2",
    "ember-template-compiler": "2.16.2",
    "ember-testing": "2.16.2"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}