Well, I got the Samsung dock, and as much as I like the E4GT, the dock is terrible. Here's my experience:
1. There's a ton of line noise when the phone is docked but nothing is playing, just like with the Epic. I'm using the OEM charger, but that's not the problem--using the same charger, the Photon's car dock is absolutely silent. Samsung must be using some cheap electronics in this thing, and it's simply horrible. (Sorry for venting, but seriously...)
2. The software isn't nearly as seamless as the Photon's either. First, on the Photon, when you dock an app starts that essentially takes over the home button. That means you can move back and forth between the app and the home screens easily. On the Samsung, it seems like once you leave the Vlingo app to, say start a radio streaming app, you can't get back to the dock mode. At least, not without unplugging/replugging the dock connector, but that's extra wear and tear. If I'm wrong about this, please let me know.
Then, the Photon app allows for 4 custom apps to be assigned. That provides a great deal of flexibility, and again it's easy to go from one app to another and then back to dock mode.
Overall, the Samsung car dock experience is nowhere near the Photon's. Motorola really did a great job with this, and Samsung makes it look like they gave actually using the phone in the car zero thought.
What's everyone else doing to make this experience more usable? I'm still choosing between the Photon and the E4GT, and the car dock definitely is a mark in the Photon's favor.