I've got an update - after about 10 hours of driving this weekend, I'm very pleased with the dock. I did go back and try the built in speaker jack, and the audio quality was just fine. I'm not sure what was up the first time I tried it (likely something stupid on my part), but the audio played fine from the dock right to my aux-in.
So long story short, the dock is great for my needs. I don't use a case or anything, so that's not an issue, and everything else works as expected. The only minor issue I've found is that when you un-dock the phone, it doesn't register as disconnecting the speakers, so you have to manually pause any music that's playing. When I use the headphone jack, it pauses the music automatically. With everything else that works flawlessly, I can overlook this minor issue.