The HS9 helps a lot, but the cover is flexible enough to allow the button to be pushed.
We are promised more options - I would like to see an option for just the power button as it is the hardest to press by accident. I have often found myself skipping tracks whilst taking the X5/HS9 out of my pocket, because I've inadvertently gripped it by the volume buttons.
Yup, I love the HS9 for general protection but the front flap does flex and you can accidentally cause the X5 to play, pause, skip, etc. when grabbing it where those controls are. I was surprised by this and have learned not to grab it in that area if I can help it.