All right, a week into the game.
GPS is still broken
Not only it is broken, the Internet is full of copy & paste of a bogus advice on how to improve it (bottomline: MS Based GPS, which, let's be honest, is not quite that GPS).
Worst of all, Samsung is mum on the issue. Had they said or did something along the lines of what Motorola did when the screen issue with Droid X was discovered, I'd calmly wait until they either issue a firmware fix or replace the phone. However, nothing happened, and, as a result, both phones will most probably be returned before the end of 14 days grace period. Maybe people returning phones will teach Samsung what the value of listening to the customer is.
Historical note: back at the beginning G1's GPS performance was as bad if not worse. It did get fixed, though, and today it is a very usable and reliable GPS source. Too bad if Vibrant's GPS problems are of software nature.
Car charger does not produce enough power to keep the battery up when Navigation is active
Yes, you've read that right. The battery charge goes down, even though the phone reports itself as "charging". A deal breaker for heavy Navigation users.
The phone reboots the UI when GPS activity is present
MyTracks is particularly vulnerable to this problem. Start recording a track, walk around, you're very likely to have the phone partially rebooted in a few minutes. Those Linux enabled will realize that it's the UI layer that is getting restarted.
GPS activity causes the phone to lock upAgain, start MyTracks and watch the phone freeze, sometimes for tens of seconds at a time.
Multiple SD Card Issues
At least under Windows XP (hasn't tried Linux yet, but that's kinda irrelevant), connecting the phone to the computer produces two drives, both of which report "Please insert disk into Drive NN" when you try to click on them.
After every other reboot, or an attempt to mount drives to a computer, the Media Scanner runs and in a short while reports either "Blank External SD Card" or "External SD Card Damaged". Of course it isn't - a reboot or plugging/unplugging the USB cable fixes the issue. Half of the time.