I've used the Mountek nGroove MT5000 for years. It's a universal CD slot mount, fully adjustable for any phone really...
I drive around for work every day for hours and hours.
Downsides... not an actual "Dock" for the M8, rather a holder for the phone. But I plug in with USB anyway to ciglighter
Upsides... Not mounted in Window, so it's in less sunlight than a windshield mount. If you're lucky, you may have a vent near the CD slot so air can flow directly to the phone if you drive for more than an hour.
+1 for the Mountek nGroove. It works amazingly well, very simple yet well made.
True, its not a "dock" if the OP is looking for something that will connect the phone for charging or connect to head unit.
Only downsides I will add, one should look at the photos of the product before buying, and determine if this holder will interfere or block any controls or displays. For instance, on a few cars, the CD slot is located low on the dash. So it might not be an ideal location for viewing the phone for navigation, etc.; or even interfere with your gearshift knob. But I travel for work a good amount, used this holder on a number of different rental car models, and it works much more often than it doesn't. And it takes about 10 seconds to install on just about any car.
Other downside, if is you actually use your CD player. Mine hasn't had a CD inserted in it for years.
The nGroove is so ridiculously simple and effective that my wife wanted one for herself immediately (so I got her one); and numerous coworkers and friends have commented how great an idea it as, and that they wanted one, too.