I think we are in agreement - I didn't word my comment well.
When I was referring to the star rating, I meant the overall average rating of the product (ie what Amazon is trying to promote now over star rating plus written reviews).
When I mentioned the negative reviews, I was referring to the 1 and 2-star written reviews. I use the negative written reviews exactly how you described - and it's what I find valuable.
I mistakenly did not see the whole OP comment when I replied. This was my fault as I'm unused to using HN on mobile and did not do my due diligence to see the full comment.
A useful heuristic that I also use, but it's good to be aware that fake reviews go in the other direction, too, of sabotaging competitors' product reviews.
plus with some of the low star reviews you can compare what they stated was wrong with your own expectations of what could go wrong with the product.
the only two considerations for me are the number of reviews and the quality of the low star reviews for the same. the dates of reviews is very useful as well, if a product doesn't have many recent good reviews it can offset the number of reviews in my view
4/5 star reviews are useless, but 2/1 star reviews reveal a great deal of useful information about a product.
A bunch of 1 star reviews of "UPS damaged my product" indicates a product that's as advertised and isn't astroturfed (much?).
No 1 star reviews, or 1 star reviews indicating misleading advertising tend to indicate astroturing.
1 star reviews indicating product failures, support failures, DOA etc... indicate to not purchase the product at all.
It's not a pure indicator, but I've found it to be more reliable than other review systems.