Works great. Not just for Amazon. I've found this to work on several other shopping websites. Recently, I was researching dell's website for a new laptop, and it worked on their site too.

Only works sometimes and seems to be affecting other sorting options.

It works! This was so needed, since Amazon removed the capability to do it natively. Thank you to both the original author of the code as well as to Boxframes for publishing this extension!

Hint: I previously thought it didn't work, but I was using it incorrectly. One must narrow their search down to a category. After that, selecting "Number of reviews" on the dropdown box properly sorts the number of items.

New option shows in Sort menu on Amazon pages.
But it seems to result in random sort order.
Sorting by Relevance is much better bet, as sorts somewhat by # of reviews (and far more often/accurately than this does).