I wanted this as I have an IGrip car stand and the location of the charger jack doesn't work with the little tabs it has at the bottom of the grip when in vertical position. As they are too close to the stand, it forces the phone up in the stand and it blocks access to the usb port. I have turned the phone horizontal so that it is better supported and removed the little plastic base pieces so I can have it charging when in the car.
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Stuntman, thanks for the link to the Grip-it holder. Mine is similar but that one will work well for my girlfriend so I'm off to pick it up. Mounting it on the air vent might be a concern though. That extra heat does not play well with electronics.
Universal stands will have this issue with some phones as they cannot account for all possible locations of the various ports where you plug cables in. The Bracketron that I use worked well for my old Nokia N97. For my Desire Z, I have had to make a few modifications to the mount.
As is, the strength of the grip is not strong enough to keep the Desire Z in place if I accelerate quickly. It fell out a couple of times. I had to cut grooves into the grip so that it wraps around the front of the phone a bit. It is easy to cut the grooves because the grips is made of a foamy material that is easy to shape.
The micro USB port is slightly hidden behind the grip. If I use my car charger, it does cause the phone to ride up a little. It doesn't bother me that much. I can fix it by putting a folded piece of paper beneath the phone.
The Bracketron is completely solid on the back of the phone. It would deflect some of the heat away from the vent. The winters aren't as cold here in Vancouver. We probably only have a dozen or so days at most where the temperature drops below freezing all day. I do turn on the heater in the winter although I usually keep the fan on the lower settings.
I personally thought it would be an issue in the summer as it would block the AC. This past summer, I didn't have any issues and the car was able to cool down as quickly as without the Bracketron the previous summer. There is some clearence between the vent and the phone, so the vent is not entirely blocked off. The air would just be deflected somewhat.
The good thing about the Bracketron is that you can mount your phone in both the horizontal and vertical position. The joint is a ball and socket joint that can rotate to any angle you want.