You've declared the name of the dependent property as the name of the computed property here (and elsewhere in this PR).  Based on your code, I would expect the `user_id` property to be declared here, like:
```js
guestCheckout: function() {
  return !this.get('user_id');
}.property('user_id')
```
also, for a simple inverse boolean computed like this, you could use `Ember.computed.not`