I've triggered and released each button on both my trackpad and Wacom tablet with this test: http://codepen.io/appsforartists/pen/NRdLrv

Here are the results

| Input (pointerdown)        | event.pointerType | event.buttons | event.pointerId | event.pressure |
| -------------------------- | ----------------- | ------------- | --------------- | -------------- |
| Wacom tap                  | mouse             | 1             | 1               | 1              |
| Wacom stylus tip           | pen               | 1             | 2               | 0.521656865596 |
| Wacom stylus lower button  | pen               | 2             | 2               | 0              |
| Wacom stylus upper button  | mouse             | 4             | 1               | 1              |
| Wacom stylus eraser        | pen               | 1             | 2               | 0.356490206480 |
| MacBook one-finger click   | mouse             | 1             | 1               | 1              |
| MacBook two-finger click   | mouse             | 2             | 1               | 0              |

Notice that for event.buttons==2 on both my trackpad and tablet, event.pressure is 0.  This should be consistent with the other button presses (currently specified to .5, but that may change in https://github.com/w3c/pointerevents/issues/147).