A little over a year ago, I purchased an expensive Panasonic LM-BE50DE dual-layer rewritable Blu-ray disc. This 50 GB media cost me $50 from Amazon. It worked fine, so I posted a positive review, which has since received 3 helpful votes.
A year later, the disc has failed. And Panasonic won't replace it because they only have a 90 days warranty on parts, which includes media !
I attempted to change my Amazon review's star rating down from 4 stars to 1 star. But it was impossible. I could only change the headline and text of the review, but not the star rating. I called Amazon about this, and was told that this could only be done within 30 days of a review ! I think that makes Amazon reviews significantly less helpful if long-term owners of a product cannot update their reviews when they later encounter problems with them.
To be fair to Amazon, it appears that it's possible to delete the review, and post it again. I haven't tried that. But doing so would void the helpful votes that my original review received.