What we tested
We used the mirror during ten-minute putting blocks and tracked start-line misses from six feet on a flat indoor mat.
Performance
The mirror did what a good training aid should do: make a hidden problem obvious. Eye-line and shoulder alignment problems became easier to correct without adding complexity.
Verdict
This is not glamorous gear, but it is useful. If your face angle changes every session, a putting mirror is a cheap way to make practice more honest.