Product review (5 stars): Great product, but install it correctly the first time!
I was reluctant about buying a 3rd-party suction cup shower head holder, to replace the factory one that finally gave up after ~3 years or so, but the SharkSPA holder's product description really emphasized its suction ability.  Too many times, we were woken from a sound sleep, or alarmed in another part of the house, by the sound of the shower head holder and the portable shower head falling and clanking in the shower.  I gave the SharkSPA unit a try, and it's worked awesomely.  But details matter, and I hope to save folks a headache with the following detail.

Within 10 minutes of initial installation (a snap), the shower head holder dropped off the shower wall - CLANK.  I set it up again in the same spot, making sure all surfaces were clean, and sure enough, it fell within 5 minutes.  I gave up and contacted the seller.  The seller had me photo the area I had placed the unit in, and ultimately decided to send me a replacement unit.  I was skeptical that it would work any better than the first unit, but thoroughly cleaned the area in the shower stall... and happened to look up at the ceiling.

The top edge of the shower insert made it easy to see that there was a *very* slight curve/contour to the insert, where I was placing the SharkSPA shower head holder.  It was so subtle that I never saw it when looking directly at the place where I was mounting the holder, and certainly couldn't feel the curvature. That little bit of contour was what was preventing the suction cup from making a perfect seal.

I re-mounted the shower head holder about 2-3" away from the curved/sloped part of the shower insert, and it has been absolutely stable and solid ever since.  I found that I could hang the shower head on it indelicately, and even pull on the holder with my hand, and the holder has not budged.  I am sharing this review so others can truly appreciate this product in the manner it was meant to be used - on a perfectly flat surface.  The SharkSPA shower can't work on contoured or curved surfaces, so please choose its placement carefully.  Do that, and you'll really come to appreciate it.