Akin to most Amazon products with multiple reviews, there's something to be gleaned both from the positive AND negative feedback. This is certainly one of thosse products.
First things first: the Aquasana Shower Filter does what it promises. Expect noticeably cleaner and fresher water immediately after installation. I've used this in two different states over a 1.5 year-period. In both instances, the natural odor of the local tap water disappeared completely from my shower after installing this filter, likewise the water gunk ring that used to form after a week or two without it.
Related, this is one wonderful showerhead. I'm amazed with all of the filtration going on (alluded to below), such powerful pressure is produced. The multiple setting showerhead is likewise quite user-friendly, rarely -- once every 8-10 months -- requiring a moderate pore cleaning.
The Aquasana Shower Filter, however, is not perfect. This is one expensive solution to shower filtration. Ditto on filters which cost upwards of $35 (minus shipping), only available online (in most instances). It also weighs a ton + is enormous; if you have old or fragile plumbing, this is NOT the filter for you. Related, shower plumbing with minimal distance between the wall and the location of the shower head (<8 inches) might encounter problems with spacing.
Installation is likewise frustrating, requiring trial and error beyond description (for a moderate DIY'er). Getting the filter on without leaking plus the shower head firmly secure to the plumbing requires sufficient patience. Based on feedback from several reviewers, special caution should also be taken not to overtighten the filters to the shower head (to avoid housing unit cracking).
It also takes a hefty number of water gallons to clean the filter's carbon residue upon initial installation. I often thought I could fill the Erie Canal in the amount of water required to completely flush one Aquasana filter.
In sum: the Aquasana filter might be expensive, an installation bugger, and both large and bulky. It does, however, work quite well. My wife and I easily notice the quality loss (in water hardness, softness, odor, ph, chemicals, etc.) every time we shower elsewhere.