Coincidentally, I was going to write up a post today recommending the Kenu Airframe as a car dock for the Nexus 5. It fits the Nexus 5 in an OEM bumper case shockingly well, darn near perfectly. I've had the dock for months and used it with an iPod, but I never tried it with my phone until today. The Airframe clips to a vent in your car pretty securely (there are differently-sized prongs that rotate, so you can mount your phone to a vent vertically or horizontally) and a spring-load clamp keeps the phone in the assembly. The nice thing is that if you need to use the phone, you just pull it off the vent with the Airframe still on it, then put it back. It's really unintrusive too; you can easily plug in a charger.
Now this is one of those docks that clips to a vent in your car, so if that isn't your preference, look elsewhere. I personally wanted something that mounted on a vent or in a cupholder after going through quite a few suction-cup mounted docks in the past few years (especially with my OG Moto DROID). I won't put a mounting plate on my dash, so I've had to stick suction-cup docks to my windshield. Inevitably, the suction cup becomes loose and the dock falls down, especially in cold weather.
The Airframe is a simple, quick little dock that works. It fits the Nexus 5 in its OEM bumper case shockingly well, it doesn't scratch up the phone, and doesn't fall down.