Not yet, but it's getting a little crowded with two iphones in the front and an ipad in the back. We usually slip the ipad into the seat-back pocket (another thing I prefer about the QP). The windshield is pretty steep for a suction cup mount, and the rearview mirror already takes up about a third of the available space. I think it would have to be a vent-mounted holder, or maybe a cup holder-type mount. My biggest gripe about most of the phone holders is that they are overly-engineered, and often dwarf the phone itself.
I should explain that I've been putting a lot of thought into this since I got a dashcam for the car. All the technology packed into the rearview mirror makes it hard to find a spot to mount the camera that won't obscure some other function. Looking around at other cars, I'm struck by what a massive structure the whole rearview mirror assembly is.