I used them before, i had two of them wired into the electrical system, i placed them behind the fixed rear quarter glass (the glass between C and D pillars), on the top of the some kind of vent (in reality it is just a grille, no vent there), stuck on the suction cups.

Those chargers will not charge the battery into a full state, they will barely keep up with the discharge that is happening with the locked and alarmed car, assuming that the panels are in broad daylight and perpendicular to the sunlight - in my case, both of the panels were at an angle to the light (almost vertical) behind the factory tinted glass (cold package). The fact that there were two of them, on either side of the car, kind of helped... If anything, it would buy you an extra day of the parking before battery goes dead...

I installed them because i already had the panels, and the install was simple with no immediate side effects...

At one point, i was thinking about mounting them on the sliding cover of the sunroof (still tinted glass but at least perpendicular to the sunlight)