Have you tried using a car charger and plugging it in while navigating?...
Since you've asked, yes, I did.
In California, the law is that anything attached to your windshield has to be in the lower left hand corner. Which is, therefore, where I put my car cradle, so I can read the nav when I'm driving. The car charger, however, plugs in the center console. When I tried to do this (needed a cord long enough to make it), the cord was dragged over the steering column, right turns pulled on the wires, and the whole thing was unsafe. Logically, it would make sense to attach the phone to the center of the dash, right above where the charger plugs in, but that's against CA law.
Are there workarounds? Sure. I can buy a different cradle and just sit it on the center of my dashboard, rather than suctioning it to the inside of my windshield. I could buy some little clamps and do a nice, tidy job of running an extra long wire around my dashboard, so that it is always available to plug in the phone while in the car cradle and not being dragged across the steering wheel. I don't like either of those workarounds while I have the perfectly good workaround of "oh, look, a spare battery in my purse." Which is what annoys me about this plan with the GS6, because I'm one of the people who really likes having a removable battery. I understand that's not an issue for everyone. Samsung is betting on it not being a significant issue for most people. With respect, nobody here is going to convince me that it's not an issue FOR ME. I respect that it's not an issue for a lot of you, but I have to confess to a certain amount of annoyance at posts (not necessarily here) that keep saying it's no big deal and we should just get over it. Everyone's usage profile is a little different -- hell, that's the great thing about Android, we can personalize the crap out of these things -- and for me, being able to yank the battery is something that really, really matters. Lots.
At this point, I'm going to just hang on to my GS4 until either (a) a good alternative with a replaceable battery appears; or (b) other advantages of a phone with a non-replaceable battery outweigh the inconvenience of losing that one feature.