Having used ng2 in production for a while, I personally dislike the model behind this, mainly because it is not actually symmetric as it appears from a first look:

- input:

  ```
  <foo [x]="y"></foo>
  ```

- output:

  ```
  <foo (x)="y = $event"></foo>
  ```

To me this seems unfinished and inelegant; note also that everything is inherently mutable, but change detection will work best when the bound input values are reassigned, but if they are instead mutated at some deeper level, Angular will have trouble tracking those changes, and may behave in weird ways, which are often tricky to debug.