A star rating feels like a verdict, but it rarely tells you what happened. Two reviewers can give the same score for reasons that have nothing to do with each other. The useful part of a review is the detail underneath the stars, so it pays to read for that.
Check whether the reviewer describes the water
Good reviews say what the water problem was: a taste or odor, staining, hardness that left deposits on fixtures, or a result from a water test. A review that never mentions the original problem is hard to compare with your own. If your problem is different, the reviewer's result may not apply to you.
Check whether the reviewer describes the install
Find out whether the reviewer had the system installed by a professional or installed it personally, and whether the install was in a home similar to yours. Installation quality often explains more of a complaint than the equipment does.
Check the time frame
A review written in the first weeks describes a new system. Reviews that mention months or years of use describe something different. Look for reviews that say how long the system had been running and whether maintenance was done on schedule.
Look for repeated, specific problems
One dramatic complaint means little. Several reviews that describe the same specific failure, in the same part of the system, deserve more attention. Also note how the seller responded. A calm, specific reply that names a fix is more useful than a generic apology.
Treat suspicious patterns with caution
Be careful when many reviews arrive in a short cluster, use nearly identical wording, or avoid any mention of the home or water involved. These patterns make it hard to know what actually happened.
Compare on your own criteria
Decide your criteria before you read. Use your own water report, the install needs of your home, and the service terms you can get in writing. Then see whether the reviews answer those questions. For how we weigh different kinds of information when we assign grades, see How Grades Work: Criteria, Weights, and the A to F Scale.