Can I use wordpress comments system to build a user review system within my website?

You can use comments and store individual ratings using add_comment_meta().

You would also probably want to store the rating average using add_post_meta(), since it’s rather expensive to calculate it on the fly. Each time a rating is added or removed, you will have to update this average.

I don’t know if there are any plugins that do this already; haven’t searched.

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