I have a Compactenna SCAN-III that I got to try on my wife's Jeep Grand Cherokee because she was concerned about anything blocking her view and needed to be able to easily still get into / out of a parking garage and home garage without jumping out every time she wanted to park. We personally didn't think it worked as well as the Comet 2x4SR that we installed on a lip mount to her hatch. We put the mount low on the hatch, around the brake light so that the top of the antenna only adds maybe 1.25' to the vehicle height.
Visually, it's like a tall aerosol can in the 24-30oz realm which I personally think is more distracting than a thin whip antenna. We also tried mounting it on the hood of her Jeep on both the driver and passenger side as shown on that pickup above and it was massively obnoxious and made 4 way stops seem dangerous to my wife. I wouldn't be surprised to get pulled over by a cop thinking you had obstructed view.
That isn't to say the CompactTenna isn't good, but it did not outperform my personal favorite mobile antenna as we were able to configure them for our needs, on her Jeep. I also tried the Compactenna on a tall tripod mount using the counterpoise kit they sell as a base antenna, and it also doesn't not quite seem to be as effective as the Comet on the same setup with the counterpoise. I personlly need more time to keep playing with it, but I don't think it's the magic bullet that many people want it to be, or the particular nature of an optimal for this antenna eludes me (and others based on online feedback).
YMMV