I have the car dock for my Atrix and find it quite handy. It keeps the phone in view at all times (obviously) so you're not fumbling around for it and keeps it charged (assuming you have it plugged in to a usb charger). I've used the Android Nav app a few times, and Pandora internet radio a lot with it, and it works especially well if you have it plugged into an auxillary jack in the car stereo. Turn by turn directions are piped into the car stereo this way, as is music from Pandora.
The car dock itself could be improved by having a mic in it for hands-free calling, and the dock software doesn't default to putting in-call sound through the car stereo speakers if connected through an aux jack to the stereo. To accomplish this, you actually have to answer the call and touch the "Speaker" option in-call to take the audio off the phone's external speaker and play it through the car speakers. Weird, but I guess not everyone has their dock connected to the car stereo.
All in all, I feel if you have an Atrix and spend a lot of time in your car, the car dock is a no brainer.
I am currently using the HD Multimedia dock and it shows the Digital Clock. It's the only dock I have and use, it's perfect for my home office enviroment. I have it attached to a 23 inch flat screen at the end of my desk and it takes up minimum space.
No dock, I currently have the bluetooth Motorola keyboard (the one for the Atrix) and works great. I was planning on buying the media dock, but since it requires tethering not willing to lose my unlimited yet .